Bulk Deletion

I am attempting to perform my annual deletion of old records.  When I select my users who have not accessed the site in 2018, and who have not completed our programs, I receive the following error for deletion.  Hopefully someone can assist with this issue:

 
Can not find data record in database table course.

More information about this error
×Debug info: SELECT id,category FROM {course} WHERE id IS NULL
[array (
)]
Error code: invalidrecord
×Stack trace: * line 1546 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown

• line 1522 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
• line 6733 of /lib/accesslib.php: call to moodle_database->get_record()
• line 70 of /lib/classes/event/grade_deleted.php: call to context_course::instance()
• line 1119 of /lib/grade/grade_grade.php: call to core\event\grade_deleted::create_from_grade()
• line 1532 of /lib/gradelib.php: call to grade_grade->delete()
• line 4154 of /lib/moodlelib.php: call to grade_user_delete()
• line 29 of /admin/user/user_bulk_delete.php: call to delete_user()