diff --git a/readme.md b/readme.md
index f93a43f..e291596 100644
--- a/readme.md
+++ b/readme.md
@@ -314,3 +314,18 @@ $schema = Expect::arrayOf('string')
 $processor->process($schema, ['a', 'b']); // it passes, 2 is even number
 $processor->process($schema, ['a', 'b', 'c']); // error, 3 is not even number
 ```
+
+Or
+
+```php
+$schema = Expect::string()->assert('is_file'); // file must exist
+```
+
+You can add custom description for every assert. This description will be part of error message.
+
+```php
+$schema = Expect::arrayOf('string')
+	->assert(function ($v) { return count($v) % 2 === 0; }, 'Even items in array');
+
+$processor->process($schema, ['a', 'b', 'c']); // Failed assertion "Even items in array" for option with value array.
+```
diff --git a/src/Schema/Elements/Base.php b/src/Schema/Elements/Base.php
index a02b09a..f5f653e 100644
--- a/src/Schema/Elements/Base.php
+++ b/src/Schema/Elements/Base.php
@@ -27,7 +27,7 @@ trait Base
 	/** @var callable|null */
 	private $before;
 
-	/** @var callable[] */
+	/** @var array[] */
 	private $asserts = [];
 
 	/** @var string|null */
@@ -62,9 +62,9 @@ public function castTo(string $type): self
 	}
 
 
-	public function assert(callable $handler): self
+	public function assert(callable $handler, string $description = null): self
 	{
-		$this->asserts[] = $handler;
+		$this->asserts[] = [$handler, $description];
 		return $this;
 	}
 
@@ -110,9 +110,9 @@ private function doFinalize($value, Context $context)
 			}
 		}
 
-		foreach ($this->asserts as $i => $assert) {
-			if (!$assert($value)) {
-				$expected = is_string($assert) ? "$assert()" : "#$i";
+		foreach ($this->asserts as $i => [$handler, $description]) {
+			if (!$handler($value)) {
+				$expected = $description ? ('"' . $description . '"') : (is_string($handler) ? "$handler()" : "#$i");
 				$context->addError("Failed assertion $expected for option %path% with value " . static::formatValue($value) . '.');
 				return;
 			}
diff --git a/src/Schema/Elements/Type.php b/src/Schema/Elements/Type.php
index f4586d9..67bdebe 100644
--- a/src/Schema/Elements/Type.php
+++ b/src/Schema/Elements/Type.php
@@ -135,8 +135,13 @@ public function complete($value, Context $context)
 			$value = []; // is unable to distinguish null from array in NEON
 		}
 
+		$cleanValue = $value;
+		if (is_array($cleanValue) && isset($cleanValue[Helpers::PREVENT_MERGING])) {
+			unset($cleanValue[Helpers::PREVENT_MERGING]);
+		}
+
 		$expected = $this->type . ($this->range === [null, null] ? '' : ':' . implode('..', $this->range));
-		if (!$this->doValidate($value, $expected, $context)) {
+		if (!$this->doValidate($cleanValue, $expected, $context)) {
 			return;
 		}
 		if ($this->pattern !== null && !preg_match("\x01^(?:$this->pattern)$\x01Du", $value)) {
@@ -150,7 +155,7 @@ public function complete($value, Context $context)
 
 		if ($this->items) {
 			$errCount = count($context->errors);
-			foreach ($value as $key => $val) {
+			foreach ($cleanValue as $key => $val) {
 				$context->path[] = $key;
 				$value[$key] = $this->items->complete($val, $context);
 				array_pop($context->path);
diff --git a/tests/Schema/Expect.assert.phpt b/tests/Schema/Expect.assert.phpt
index 7d419bc..49d23c8 100644
--- a/tests/Schema/Expect.assert.phpt
+++ b/tests/Schema/Expect.assert.phpt
@@ -34,3 +34,20 @@ test(function () { // multiple assertions
 
 	Assert::same('123', (new Processor)->process($schema, '123'));
 });
+
+
+test(function () { // multiple assertions with custom descriptions
+	$schema = Expect::string()
+		->assert('ctype_digit', 'Is number')
+		->assert(function ($s) { return strlen($s) >= 3; }, 'Minimal lenght');
+
+	checkValidationErrors(function () use ($schema) {
+		(new Processor)->process($schema, '');
+	}, ["Failed assertion \"Is number\" for option with value ''."]);
+
+	checkValidationErrors(function () use ($schema) {
+		(new Processor)->process($schema, '1');
+	}, ["Failed assertion \"Minimal lenght\" for option with value '1'."]);
+
+	Assert::same('123', (new Processor)->process($schema, '123'));
+});
diff --git a/tests/Schema/Expect.list.phpt b/tests/Schema/Expect.list.phpt
index e58bac2..d8175bd 100644
--- a/tests/Schema/Expect.list.phpt
+++ b/tests/Schema/Expect.list.phpt
@@ -3,6 +3,7 @@
 declare(strict_types=1);
 
 use Nette\Schema\Expect;
+use Nette\Schema\Helpers;
 use Nette\Schema\Processor;
 use Tester\Assert;
 
@@ -45,6 +46,8 @@ test(function () { // merging
 	Assert::same([1, 2, 3, 'a', 'b', 'c'], (new Processor)->process($schema, ['a', 'b', 'c']));
 
 	Assert::same([1, 2, 3], (new Processor)->process($schema, null));
+
+	Assert::same(['a', 'b', 'c'], (new Processor)->process($schema, ['a', 'b', 'c', Helpers::PREVENT_MERGING => true]));
 });
 
 
@@ -64,6 +67,8 @@ test(function () { // merging & other items validation
 	]);
 
 	Assert::same([1, 2, 3], (new Processor)->process($schema, null));
+
+	Assert::same(['a', 'b', 'c'], (new Processor)->process($schema, ['a', 'b', 'c', Helpers::PREVENT_MERGING => true]));
 });