diff --git a/src/Mapper/Object/Argument.php b/src/Mapper/Object/Argument.php
index df948900..0b372ee5 100644
--- a/src/Mapper/Object/Argument.php
+++ b/src/Mapper/Object/Argument.php
@@ -15,19 +15,41 @@ final class Argument
     private Type $type;
 
     /** @var mixed */
-    private $value;
+    private $defaultValue;
 
-    private ?Attributes $attributes;
+    private bool $isRequired = true;
 
-    /**
-     * @param mixed $value
-     */
-    public function __construct(string $name, Type $type, $value, Attributes $attributes = null)
+    private Attributes $attributes;
+
+    private function __construct(string $name, Type $type)
     {
         $this->name = $name;
         $this->type = $type;
-        $this->value = $value;
-        $this->attributes = $attributes;
+    }
+
+    public static function required(string $name, Type $type): self
+    {
+        return new self($name, $type);
+    }
+
+    /**
+     * @param mixed $defaultValue
+     */
+    public static function optional(string $name, Type $type, $defaultValue): self
+    {
+        $instance = new self($name, $type);
+        $instance->defaultValue = $defaultValue;
+        $instance->isRequired = false;
+
+        return $instance;
+    }
+
+    public function withAttributes(Attributes $attributes): self
+    {
+        $clone = clone $this;
+        $clone->attributes = $attributes;
+
+        return $clone;
     }
 
     public function name(): string
@@ -43,9 +65,14 @@ public function type(): Type
     /**
      * @return mixed
      */
-    public function value()
+    public function defaultValue()
+    {
+        return $this->defaultValue;
+    }
+
+    public function isRequired(): bool
     {
-        return $this->value;
+        return $this->isRequired;
     }
 
     public function attributes(): Attributes
diff --git a/src/Mapper/Object/DateTimeObjectBuilder.php b/src/Mapper/Object/DateTimeObjectBuilder.php
index 11214800..cca508b1 100644
--- a/src/Mapper/Object/DateTimeObjectBuilder.php
+++ b/src/Mapper/Object/DateTimeObjectBuilder.php
@@ -5,9 +5,13 @@
 namespace CuyZ\Valinor\Mapper\Object;
 
 use CuyZ\Valinor\Mapper\Object\Exception\CannotParseToDateTime;
+use CuyZ\Valinor\Type\Type;
 use CuyZ\Valinor\Type\Types\NonEmptyStringType;
 use CuyZ\Valinor\Type\Types\NullType;
 use CuyZ\Valinor\Type\Types\PositiveIntegerType;
+use CuyZ\Valinor\Type\Types\ShapedArrayElement;
+use CuyZ\Valinor\Type\Types\ShapedArrayType;
+use CuyZ\Valinor\Type\Types\StringValueType;
 use CuyZ\Valinor\Type\Types\UnionType;
 use DateTime;
 use DateTimeImmutable;
@@ -24,6 +28,8 @@ final class DateTimeObjectBuilder implements ObjectBuilder
     public const DATE_MYSQL = 'Y-m-d H:i:s';
     public const DATE_PGSQL = 'Y-m-d H:i:s.u';
 
+    private static Type $argumentType;
+
     /** @var class-string<DateTime|DateTimeImmutable> */
     private string $className;
 
@@ -35,24 +41,35 @@ public function __construct(string $className)
         $this->className = $className;
     }
 
-    public function describeArguments($source): iterable
+    public function describeArguments(): iterable
     {
-        $datetime = $source;
-        $format = null;
-
-        if (is_array($source)) {
-            $datetime = $source['datetime'] ?? null;
-            $format = $source['format'] ?? null;
-        }
-
-        yield new Argument('datetime', new UnionType(PositiveIntegerType::get(), NonEmptyStringType::get()), $datetime);
-        yield new Argument('format', new UnionType(NullType::get(), NonEmptyStringType::get()), $format);
+        self::$argumentType ??= new UnionType(
+            new UnionType(PositiveIntegerType::get(), NonEmptyStringType::get()),
+            new ShapedArrayType(
+                new ShapedArrayElement(
+                    new StringValueType('datetime'),
+                    new UnionType(PositiveIntegerType::get(), NonEmptyStringType::get())
+                ),
+                new ShapedArrayElement(
+                    new StringValueType('format'),
+                    new UnionType(NullType::get(), NonEmptyStringType::get()),
+                    true
+                ),
+            )
+        );
+
+        yield Argument::required('value', self::$argumentType);
     }
 
     public function build(array $arguments): DateTimeInterface
     {
-        $datetime = $arguments['datetime'];
-        $format = $arguments['format'];
+        $datetime = $arguments['value'];
+        $format = null;
+
+        if (is_array($datetime)) {
+            $format = $datetime['format'];
+            $datetime = $datetime['datetime'];
+        }
 
         assert(is_string($datetime) || is_int($datetime));
         assert(is_string($format) || is_null($format));
diff --git a/src/Mapper/Object/MethodObjectBuilder.php b/src/Mapper/Object/MethodObjectBuilder.php
index 1d774978..386543e8 100644
--- a/src/Mapper/Object/MethodObjectBuilder.php
+++ b/src/Mapper/Object/MethodObjectBuilder.php
@@ -10,21 +10,14 @@
 use CuyZ\Valinor\Mapper\Object\Exception\ConstructorMethodIsNotStatic;
 use CuyZ\Valinor\Mapper\Object\Exception\InvalidConstructorMethodClassReturnType;
 use CuyZ\Valinor\Mapper\Object\Exception\InvalidConstructorMethodReturnType;
-use CuyZ\Valinor\Mapper\Object\Exception\InvalidSourceForObject;
 use CuyZ\Valinor\Mapper\Object\Exception\MethodNotFound;
 use CuyZ\Valinor\Mapper\Object\Exception\MissingMethodArgument;
 use CuyZ\Valinor\Mapper\Tree\Message\ThrowableMessage;
 use CuyZ\Valinor\Type\Types\ClassType;
 use Exception;
 
-use function array_key_exists;
-use function array_keys;
 use function array_values;
-use function count;
 use function is_a;
-use function is_array;
-use function is_iterable;
-use function iterator_to_array;
 
 final class MethodObjectBuilder implements ObjectBuilder
 {
@@ -66,17 +59,14 @@ public function __construct(ClassDefinition $class, string $methodName)
         }
     }
 
-    public function describeArguments($source): iterable
+    public function describeArguments(): iterable
     {
-        $source = $this->transformSource($source);
-
         foreach ($this->method->parameters() as $parameter) {
-            $name = $parameter->name();
-            $type = $parameter->type();
-            $attributes = $parameter->attributes();
-            $value = array_key_exists($name, $source) ? $source[$name] : $parameter->defaultValue();
+            $argument = $parameter->isOptional()
+                ? Argument::optional($parameter->name(), $parameter->type(), $parameter->defaultValue())
+                : Argument::required($parameter->name(), $parameter->type());
 
-            yield new Argument($name, $type, $value, $attributes);
+            yield $argument->withAttributes($parameter->attributes());
         }
     }
 
@@ -92,48 +82,18 @@ public function build(array $arguments): object
         $methodName = $this->method->name();
 
         try {
+            // @PHP8.0 `array_values` can be removed
+            $arguments = array_values($arguments);
+
             if (! $this->method->isStatic()) {
-                // @PHP8.0 `array_values` can be removed
                 /** @infection-ignore-all */
-                return new $className(...array_values($arguments));
+                return new $className(...$arguments);
             }
 
-            // @PHP8.0 `array_values` can be removed
             /** @infection-ignore-all */
-            return $className::$methodName(...array_values($arguments)); // @phpstan-ignore-line
+            return $className::$methodName(...$arguments); // @phpstan-ignore-line
         } catch (Exception $exception) {
             throw ThrowableMessage::from($exception);
         }
     }
-
-    /**
-     * @param mixed $source
-     * @return mixed[]
-     */
-    private function transformSource($source): array
-    {
-        if ($source === null) {
-            return [];
-        }
-
-        if (is_iterable($source) && ! is_array($source)) {
-            $source = iterator_to_array($source);
-        }
-
-        $parameters = $this->method->parameters();
-
-        if (count($parameters) === 1) {
-            $name = array_keys(iterator_to_array($parameters))[0];
-
-            if (! is_array($source) || ! array_key_exists($name, $source)) {
-                $source = [$name => $source];
-            }
-        }
-
-        if (! is_array($source)) {
-            throw new InvalidSourceForObject($source);
-        }
-
-        return $source;
-    }
 }
diff --git a/src/Mapper/Object/ObjectBuilder.php b/src/Mapper/Object/ObjectBuilder.php
index d88cf909..4b06d555 100644
--- a/src/Mapper/Object/ObjectBuilder.php
+++ b/src/Mapper/Object/ObjectBuilder.php
@@ -7,10 +7,9 @@
 interface ObjectBuilder
 {
     /**
-     * @param mixed $source
      * @return iterable<Argument>
      */
-    public function describeArguments($source): iterable;
+    public function describeArguments(): iterable;
 
     /**
      * @param array<string, mixed> $arguments
diff --git a/src/Mapper/Object/ReflectionObjectBuilder.php b/src/Mapper/Object/ReflectionObjectBuilder.php
index 7da570d9..cb1d528e 100644
--- a/src/Mapper/Object/ReflectionObjectBuilder.php
+++ b/src/Mapper/Object/ReflectionObjectBuilder.php
@@ -5,14 +5,9 @@
 namespace CuyZ\Valinor\Mapper\Object;
 
 use CuyZ\Valinor\Definition\ClassDefinition;
-use CuyZ\Valinor\Mapper\Object\Exception\InvalidSourceForObject;
 use CuyZ\Valinor\Mapper\Object\Exception\MissingPropertyArgument;
 
 use function array_key_exists;
-use function array_keys;
-use function count;
-use function is_array;
-use function iterator_to_array;
 
 final class ReflectionObjectBuilder implements ObjectBuilder
 {
@@ -23,17 +18,14 @@ public function __construct(ClassDefinition $class)
         $this->class = $class;
     }
 
-    public function describeArguments($source): iterable
+    public function describeArguments(): iterable
     {
-        $source = $this->transformSource($source);
-
         foreach ($this->class->properties() as $property) {
-            $name = $property->name();
-            $type = $property->type();
-            $attributes = $property->attributes();
-            $value = array_key_exists($name, $source) ? $source[$name] : $property->defaultValue();
+            $argument = $property->hasDefaultValue()
+                ? Argument::optional($property->name(), $property->type(), $property->defaultValue())
+                : Argument::required($property->name(), $property->type());
 
-            yield new Argument($name, $type, $value, $attributes);
+            yield $argument->withAttributes($property->attributes());
         }
     }
 
@@ -57,35 +49,4 @@ public function build(array $arguments): object
 
         return $object;
     }
-
-    /**
-     * @param mixed $source
-     * @return mixed[]
-     */
-    private function transformSource($source): array
-    {
-        if ($source === null) {
-            return [];
-        }
-
-        if (is_iterable($source) && ! is_array($source)) {
-            $source = iterator_to_array($source);
-        }
-
-        $properties = $this->class->properties();
-
-        if (count($properties) === 1) {
-            $name = array_keys(iterator_to_array($properties))[0];
-
-            if (! is_array($source) || ! array_key_exists($name, $source)) {
-                $source = [$name => $source];
-            }
-        }
-
-        if (! is_array($source)) {
-            throw new InvalidSourceForObject($source);
-        }
-
-        return $source;
-    }
 }
diff --git a/src/Mapper/Tree/Builder/ClassNodeBuilder.php b/src/Mapper/Tree/Builder/ClassNodeBuilder.php
index c1561056..afc82643 100644
--- a/src/Mapper/Tree/Builder/ClassNodeBuilder.php
+++ b/src/Mapper/Tree/Builder/ClassNodeBuilder.php
@@ -5,13 +5,20 @@
 namespace CuyZ\Valinor\Mapper\Tree\Builder;
 
 use CuyZ\Valinor\Definition\Repository\ClassDefinitionRepository;
+use CuyZ\Valinor\Mapper\Object\Argument;
+use CuyZ\Valinor\Mapper\Object\Exception\InvalidSourceForObject;
 use CuyZ\Valinor\Mapper\Object\Factory\ObjectBuilderFactory;
 use CuyZ\Valinor\Mapper\Object\ObjectBuilder;
 use CuyZ\Valinor\Mapper\Tree\Node;
 use CuyZ\Valinor\Mapper\Tree\Shell;
 use CuyZ\Valinor\Type\Types\ClassType;
 
+use function array_key_exists;
 use function assert;
+use function count;
+use function is_array;
+use function is_iterable;
+use function iterator_to_array;
 
 final class ClassNodeBuilder implements NodeBuilder
 {
@@ -28,7 +35,7 @@ public function __construct(ClassDefinitionRepository $classDefinitionRepository
     public function build(Shell $shell, RootNodeBuilder $rootBuilder): Node
     {
         $type = $shell->type();
-        $value = $shell->value();
+        $source = $shell->value();
 
         assert($type instanceof ClassType);
 
@@ -36,10 +43,16 @@ public function build(Shell $shell, RootNodeBuilder $rootBuilder): Node
         $builder = $this->objectBuilderFactory->for($class);
 
         $children = [];
+        $arguments = [...$builder->describeArguments()];
+        $source = $this->transformSource($source, ...$arguments);
 
-        foreach ($builder->describeArguments($value) as $arg) {
-            $child = $shell->child($arg->name(), $arg->type(), $arg->value(), $arg->attributes());
+        foreach ($arguments as $argument) {
+            $name = $argument->name();
+            $type = $argument->type();
+            $attributes = $argument->attributes();
+            $value = array_key_exists($name, $source) ? $source[$name] : $argument->defaultValue();
 
+            $child = $shell->child($name, $type, $value, $attributes);
             $children[] = $rootBuilder->build($child);
         }
 
@@ -48,6 +61,35 @@ public function build(Shell $shell, RootNodeBuilder $rootBuilder): Node
         return Node::branch($shell, $object, $children);
     }
 
+    /**
+     * @param mixed $source
+     * @return mixed[]
+     */
+    private function transformSource($source, Argument ...$arguments): array
+    {
+        if ($source === null) {
+            return [];
+        }
+
+        if (is_iterable($source) && ! is_array($source)) {
+            $source = iterator_to_array($source);
+        }
+
+        if (count($arguments) === 1) {
+            $name = $arguments[0]->name();
+
+            if (! is_array($source) || ! array_key_exists($name, $source)) {
+                $source = [$name => $source];
+            }
+        }
+
+        if (! is_array($source)) {
+            throw new InvalidSourceForObject($source);
+        }
+
+        return $source;
+    }
+
     /**
      * @param Node[] $children
      */
diff --git a/tests/Fake/Mapper/Object/FakeObjectBuilder.php b/tests/Fake/Mapper/Object/FakeObjectBuilder.php
index 9bade1b8..6a2f0b26 100644
--- a/tests/Fake/Mapper/Object/FakeObjectBuilder.php
+++ b/tests/Fake/Mapper/Object/FakeObjectBuilder.php
@@ -9,7 +9,7 @@
 
 final class FakeObjectBuilder implements ObjectBuilder
 {
-    public function describeArguments($source): iterable
+    public function describeArguments(): iterable
     {
         return [];
     }
diff --git a/tests/Integration/Mapping/Object/DateTimeMappingTest.php b/tests/Integration/Mapping/Object/DateTimeMappingTest.php
index 8ae0419a..64c2068e 100644
--- a/tests/Integration/Mapping/Object/DateTimeMappingTest.php
+++ b/tests/Integration/Mapping/Object/DateTimeMappingTest.php
@@ -103,10 +103,9 @@ public function test_invalid_array_source_throws_exception(): void
                     ],
                 ]);
         } catch (MappingError $exception) {
-            $error = $exception->node()->children()['dateTime']->children()['datetime']->messages()[0];
+            $error = $exception->node()->children()['dateTime']->children()['value']->messages()[0];
 
-            self::assertSame('1618742357', $error->code());
-            self::assertSame('Cannot assign an empty value to union type `positive-int|non-empty-string`.', (string)$error);
+            self::assertSame('1607027306', $error->code());
         }
     }
 }
diff --git a/tests/Integration/Mapping/Object/ObjectValuesMappingTest.php b/tests/Integration/Mapping/Object/ObjectValuesMappingTest.php
index 89a5ca4a..dbf422ba 100644
--- a/tests/Integration/Mapping/Object/ObjectValuesMappingTest.php
+++ b/tests/Integration/Mapping/Object/ObjectValuesMappingTest.php
@@ -13,6 +13,7 @@ final class ObjectValuesMappingTest extends IntegrationTest
     public function test_values_are_mapped_properly(): void
     {
         $source = [
+            'string' => 'foo',
             'object' => [
                 'value' => 'foo',
             ],
@@ -28,17 +29,36 @@ public function test_values_are_mapped_properly(): void
             self::assertSame('foo', $result->object->value);
         }
     }
+
+    public function test_invalid_iterable_source_throws_exception(): void
+    {
+        $source = 'foo';
+
+        foreach ([ObjectValues::class, ObjectValuesWithConstructor::class] as $class) {
+            try {
+                $this->mapperBuilder->mapper()->map($class, $source);
+            } catch (MappingError $exception) {
+                $error = $exception->node()->messages()[0];
+
+                self::assertSame('1632903281', $error->code());
+                self::assertSame('Invalid source type `string`, it must be an iterable.', (string)$error);
+            }
+        }
+    }
 }
 
 class ObjectValues
 {
     public SimpleObject $object;
+
+    public string $string;
 }
 
 class ObjectValuesWithConstructor extends ObjectValues
 {
-    public function __construct(SimpleObject $object)
+    public function __construct(SimpleObject $object, string $string)
     {
         $this->object = $object;
+        $this->string = $string;
     }
 }
diff --git a/tests/Unit/Mapper/Object/MethodObjectBuilderTest.php b/tests/Unit/Mapper/Object/MethodObjectBuilderTest.php
index a50631d4..7ecd404e 100644
--- a/tests/Unit/Mapper/Object/MethodObjectBuilderTest.php
+++ b/tests/Unit/Mapper/Object/MethodObjectBuilderTest.php
@@ -4,18 +4,15 @@
 
 namespace CuyZ\Valinor\Tests\Unit\Mapper\Object;
 
-use CuyZ\Valinor\Mapper\Object\Argument;
 use CuyZ\Valinor\Mapper\Object\Exception\ConstructorMethodIsNotPublic;
 use CuyZ\Valinor\Mapper\Object\Exception\ConstructorMethodIsNotStatic;
 use CuyZ\Valinor\Mapper\Object\Exception\InvalidConstructorMethodClassReturnType;
 use CuyZ\Valinor\Mapper\Object\Exception\InvalidConstructorMethodReturnType;
-use CuyZ\Valinor\Mapper\Object\Exception\InvalidSourceForObject;
 use CuyZ\Valinor\Mapper\Object\Exception\MethodNotFound;
 use CuyZ\Valinor\Mapper\Object\Exception\MissingMethodArgument;
 use CuyZ\Valinor\Mapper\Object\MethodObjectBuilder;
 use CuyZ\Valinor\Mapper\Tree\Message\ThrowableMessage;
 use CuyZ\Valinor\Tests\Fake\Definition\FakeClassDefinition;
-use Generator;
 use PHPUnit\Framework\TestCase;
 use ReflectionClass;
 use RuntimeException;
@@ -118,26 +115,6 @@ public static function invalidConstructor(): stdClass
         new MethodObjectBuilder($class, 'invalidConstructor');
     }
 
-    public function test_invalid_source_type_throws_exception(): void
-    {
-        $object = new class () {
-            public function __construct()
-            {
-            }
-        };
-
-        $class = FakeClassDefinition::fromReflection(new ReflectionClass($object));
-        $objectBuilder = new MethodObjectBuilder($class, '__construct');
-
-        $this->expectException(InvalidSourceForObject::class);
-        $this->expectExceptionCode(1632903281);
-        $this->expectExceptionMessage('Invalid source type `string`, it must be an iterable.');
-
-        /** @var Generator<Argument> $arguments */
-        $arguments = $objectBuilder->describeArguments('foo');
-        $arguments->current();
-    }
-
     public function test_missing_arguments_throws_exception(): void
     {
         $object = new class ('foo') {
diff --git a/tests/Unit/Mapper/Object/ReflectionObjectBuilderTest.php b/tests/Unit/Mapper/Object/ReflectionObjectBuilderTest.php
index 5ff6df36..90c4d3d2 100644
--- a/tests/Unit/Mapper/Object/ReflectionObjectBuilderTest.php
+++ b/tests/Unit/Mapper/Object/ReflectionObjectBuilderTest.php
@@ -4,12 +4,9 @@
 
 namespace CuyZ\Valinor\Tests\Unit\Mapper\Object;
 
-use CuyZ\Valinor\Mapper\Object\Argument;
-use CuyZ\Valinor\Mapper\Object\Exception\InvalidSourceForObject;
 use CuyZ\Valinor\Mapper\Object\Exception\MissingPropertyArgument;
 use CuyZ\Valinor\Mapper\Object\ReflectionObjectBuilder;
 use CuyZ\Valinor\Tests\Fake\Definition\FakeClassDefinition;
-use Generator;
 use PHPUnit\Framework\TestCase;
 use ReflectionClass;
 
@@ -53,26 +50,6 @@ public function valueC(): string
         self::assertSame('valueC', $result->valueC()); // @phpstan-ignore-line
     }
 
-    public function test_invalid_source_type_throws_exception(): void
-    {
-        $object = new class () {
-            public function __construct()
-            {
-            }
-        };
-
-        $class = FakeClassDefinition::fromReflection(new ReflectionClass($object));
-        $objectBuilder = new ReflectionObjectBuilder($class);
-
-        $this->expectException(InvalidSourceForObject::class);
-        $this->expectExceptionCode(1632903281);
-        $this->expectExceptionMessage('Invalid source type `string`, it must be an iterable.');
-
-        /** @var Generator<Argument> $arguments */
-        $arguments = $objectBuilder->describeArguments('foo');
-        $arguments->current();
-    }
-
     public function test_missing_arguments_throws_exception(): void
     {
         $object = new class () {