Warning/error when attempting to edit custom post type

I have two custom post types in my WordPress installation. Both set up the same way in functions.php (one called “Media” and the other “Testimonials”). For some reason, I’m now getting the following error in Admin when I go to edit or add Media items:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'add_media_metaboxes' not found or invalid function name in /home/user/public_html/wp-includes/plugin.php on line 405

This does not affect my ability to add or edit Media items. Why is this error occurring, and how can I get rid of the warning?

The site just changed domains, but I can’t see how that would affect one custom post type and not the other.

Thanks.