hook_form_alter needs updating?

I found the hook_form_alter in customerror.module was not working properly for me: using the Search form from the Custom Error 404 page would result in the form being submitted to the Custom Error page itself, rather than to the search, which the code suggests it should be.

I solved the issue by updating:

function customerror_form_alter($form_id, &$form) {

to read, instead:

function customerror_form_alter(&$form, $form_state, $form_id) {

This solved the problem.