Error message

• Notice: Undefined index: comment_node_simplenews_form in drupal_retrieve_form() (line 750 of /home/shezaf/ofer-drupal7/includes/form.inc).
• Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'comment_node_simplenews_form' was given in drupal_retrieve_form() (line 785 of /home/shezaf/ofer-drupal7/includes/form.inc).