diff --git a/validation/custom_constraint.rst b/validation/custom_constraint.rst
index 4e947290ef0..b946901c6f8 100644
--- a/validation/custom_constraint.rst
+++ b/validation/custom_constraint.rst
@@ -65,6 +65,10 @@ The validator class is also simple, and only has one required method ``validate(
     {
         public function validate($value, Constraint $constraint)
         {
+            if (!$constraint instanceof ContainsAlphanumeric) {
+               throw new UnexpectedTypeException($constraint, ContainsAlphanumeric::class);
+            }
+            
             // custom constraints should ignore null and empty values to allow
             // other constraints (NotBlank, NotNull, etc.) take care of that
             if (null === $value || '' === $value) {