WP-MalWatch

I’ve installed WP-MalWatch, which went smoothly enough but upon looking at any page in the admin area, this warning is displayed:

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘WPMW::addAdministrativeWarning’ was given in [PATH TO WORDPRESS]/wp-includes/plugin.php on line 339

The plugin is able to scan, or at least reports back that the scan was successful.

I’m running 2.9.1 on PHP 5.

Any thoughts on what could be causing it?