/**
  * If we are using the default validation error message, make it dynamic based
  * on the allowed options.
  * @return string
  */
 public function get_validation_error_message()
 {
     $parent_validation_error_message = parent::get_validation_error_message();
     if (!$parent_validation_error_message) {
         $enum_options = $this->_input instanceof EE_Form_Input_With_Options_Base ? $this->_input->flat_options() : '';
         return sprintf(__("This is not allowed option. Allowed options are %s.", "event_espresso"), implode(', ', $enum_options));
     } else {
         return $parent_validation_error_message;
     }
 }