add a signup enable event and I get this error

I am using event and signup. I add an event with sign-up enabled, and I get:

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'signup_form' was given in /home/protective/www/www/includes/form.inc on line 366.

Any help is greatly appreciated. Thanks for all your support!!

Mike