DB read error: questionnaire_response.username should be an integer

Attempting to access the 'Recent activity' report in a course containing a Questionnaire instance results in the 'Error reading from database' message.

On investigation this turns out to be caused by a join in questionnaire_get_recent_mod_activity() on user.id = questionnaire_response.username, which have different types. As questionnaire_response.username is actually the Moodle user id it seems this should be an integer rather than a varchar.