fix bug

Author: taylorotwell

src/Illuminate/Validation/Validator.php

@@ -335,8 +335,10 @@ protected function validateAttribute($attribute, $rule)
         // attribute is invalid and we will add a failure message for this failing attribute.
         $validatable = $this->isValidatable($rule, $attribute, $value);
 
-        if ($validatable && $rule instanceof RuleContract) {
-            return $this->validateUsingCustomRule($attribute, $value, $rule);
+        if ($rule instanceof RuleContract) {
+            return $validatable
+                    ? $this->validateUsingCustomRule($attribute, $value, $rule)
+                    : null;
         }
 
         $method = "validate{$rule}";