Error on second level of admin pages: First argument is expected to be a valid callback, 'slate_system_settings_form'

While the first level looks as it should, the second does not.

I get this error

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'slate_system_settings_form' was given in /drupal/includes/theme.inc on line 617.

selected slate (formally admin) for the admin theme