diff --git a/readme.md b/readme.md
index 2191367..fdaf648 100644
--- a/readme.md
+++ b/readme.md
@@ -311,3 +311,18 @@ $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
 ```
+
+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($v) % 2 === 0; }, 'Even items in array'); // count must be even number
+
+$processor->process($schema, ['a', 'b', 'c']); // Failed assertion "Even items in array" for option with value array.
+```
+
+PHP build in functions can be used too:
+
+```php
+$schema = Expect::string()->assert('is_file');
+```
diff --git a/src/Schema/Elements/Base.php b/src/Schema/Elements/Base.php
index a02b09a..0cec71c 100644
--- a/src/Schema/Elements/Base.php
+++ b/src/Schema/Elements/Base.php
@@ -62,9 +62,13 @@ public function castTo(string $type): self
 	}
 
 
-	public function assert(callable $handler): self
+	public function assert(callable $handler, ?string $description = null): self
 	{
-		$this->asserts[] = $handler;
+		if ($description === null) {
+			$this->asserts[] = $handler;
+		} else {
+			$this->asserts[$description] = $handler;
+		}
 		return $this;
 	}
 
@@ -112,7 +116,7 @@ private function doFinalize($value, Context $context)
 
 		foreach ($this->asserts as $i => $assert) {
 			if (!$assert($value)) {
-				$expected = is_string($assert) ? "$assert()" : "#$i";
+				$expected = !is_numeric($i) ? "\"$i\"" : (is_string($assert) ? "$assert()" : "#$i");
 				$context->addError("Failed assertion $expected for option %path% with value " . static::formatValue($value) . '.');
 				return;
 			}
diff --git a/tests/Schema/Expect.assert.phpt b/tests/Schema/Expect.assert.phpt
index 7d419bc..5679daa 100644
--- a/tests/Schema/Expect.assert.phpt
+++ b/tests/Schema/Expect.assert.phpt
@@ -20,6 +20,15 @@ test(function () { // single assertion
 	Assert::same(__FILE__, (new Processor)->process($schema, __FILE__));
 });
 
+test(function () { // single assertion with custom description
+	$schema = Expect::string()->assert('is_file', 'File exists');
+
+	checkValidationErrors(function () use ($schema) {
+		(new Processor)->process($schema, 'hello');
+	}, ["Failed assertion \"File exists\" for option with value 'hello'."]);
+
+	Assert::same(__FILE__, (new Processor)->process($schema, __FILE__));
+});
 
 test(function () { // multiple assertions
 	$schema = Expect::string()->assert('ctype_digit')->assert(function ($s) { return strlen($s) >= 3; });
@@ -34,3 +43,19 @@ test(function () { // multiple assertions
 
 	Assert::same('123', (new Processor)->process($schema, '123'));
 });
+
+test(function () { // multiple assertions with custom description
+	$schema = Expect::string()
+		->assert('ctype_digit', 'Is number')
+		->assert(function ($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'));
+});