Editing and saving a forum leads to "An internal error occurred in the completion system. ....Cannot find grade item for 'forum'"

When an editing an existing a non-graded forum and saving I get the following error:

An internal error occurred in the completion system. (System administrators can enable debugging information to see more detail.)

More information about this error

Debug info: Cannot find grade item for 'forum'
cm '17959' matching number '0'
Stack trace:
line 1317 of /lib/completionlib.php: moodle_exception thrown
line 568 of /lib/completionlib.php: call to completion_info->internal_systemerror()
line 492 of /lib/completionlib.php: call to completion_info->internal_get_state()
line 789 of /lib/completionlib.php: call to completion_info->update_state()
line 342 of /course/modedit.php: call to completion_info->reset_all_state()

Notes:
1. this error does not seem to occur when creating a forum for the first time
2: although clicking on either the [Save and return to course] or [Save and display] buttons gets the above error message, changes made to the "Forum introduction" seem to be saved, but changes made to the "Forum name" do not seem to be saved.
3. the workaround is to edit forum, select an "Aggregate type" and "Scale", save, which does not lead to an error, and then edit again and change "Aggregate type" back to 'No ratings' and save again.

I'd really appreciate a fix for this. Thanks.