diff --git a/validation/custom_constraint.rst b/validation/custom_constraint.rst
index 2430143dd0e..f67b3630276 100644
--- a/validation/custom_constraint.rst
+++ b/validation/custom_constraint.rst
@@ -59,11 +59,22 @@ The validator class is also simple, and only has one required method ``validate(
 
     use Symfony\Component\Validator\Constraint;
     use Symfony\Component\Validator\ConstraintValidator;
+    use Symfony\Component\Validator\Exception\UnexpectedTypeException;
 
     class ContainsAlphanumericValidator extends ConstraintValidator
     {
         public function validate($value, Constraint $constraint)
         {
+            // custom constraints should ignore null and empty values to allow
+            // other constraints (NotBlank, NotNull, etc.) take care of that
+            if (null === $value || '' === $value) {
+                return;
+            }
+
+            if (!is_string($value)) {
+                throw new UnexpectedTypeException($value, 'string');
+            }
+
             if (!preg_match('/^[a-zA-Z0-9]+$/', $value, $matches)) {
                 // If you're using the new 2.5 validation API (you probably are!)
                 $this->context->buildViolation($constraint->message)