Error message

• Notice: Undefined index: comment_node_ad_form in drupal_retrieve_form() (line 737 of /home/rivulcom/drupal-7.4/includes/form.inc).
• Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'comment_node_ad_form' was given in drupal_retrieve_form() (line 772 of /home/rivulcom/drupal-7.4/includes/form.inc).