User services failure: user.save can't find user_profile_form

When attempting to update an existing user's profile information (a category called 'Personal Information'), I receive this error:

call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'user_profile_form' was given in /Users/aaron/Projects/cdi_drupal/6/includes/form.inc on line 372.

I believe this is because user.pages.inc is not loaded when this function runs. I've attached a patch (probably in a horribly mangled format from my Git repository) that solves the issue for me. The code is similar to the code inside node.save.