Error message for JSON validation

resources/lang/en/validation.php

@@ -41,6 +41,7 @@ return [
     'in'                   => 'The selected :attribute is invalid.',
     'integer'              => 'The :attribute must be an integer.',
     'ip'                   => 'The :attribute must be a valid IP address.',
+    'json'                 => 'The :attribute must be a valid JSON string.',
     'max'                  => [
         'numeric' => 'The :attribute may not be greater than :max.',
         'file'    => 'The :attribute may not be greater than :max kilobytes.',