The error occurs in the very specific circumstances when the select/course elements have "setDefault" instead of "set_data". The course completion form is a good example, it normally uses "select" element but it was changed to "course" element in MDL-10965 (currently in integration). To test both elements on both versions take the test file from https://gist.github.com/marinaglancy/e70f77b2830dacf45447f24e85ae3e88 . If you don't have courses with ids 2 and 3 change the "setDefault()" in the test file. Test on all themes! (boost & clean in 3.5 and 3.6, boost & classic in master) Submit the form with courses selected and without Make sure that form data (displayed after submit) always contains both 'courses' and 'cselect' elements and they correctly represent what was selected Unselect all courses from both elements. Make sure the data has two empty arrays Check "Enable cselect1 and text1" and submit the form - make sure that properties "cselect1" and "text1" are in the data object Uncheck "Enable cselect1 and text1" and submit the form - make sure that properties "cselect1" and "text1" are NOT in the data object Testing actual course completion form: Setup Have the following setup: Log in as an admin. Go to " Site administration / Advanced features " Make sure " Enable completion tracking " is ticked. Create courses A and B, both with completion tracking enabled Create any activity or resource with completion tracking in Course A. On Course A's course page, open the cog menu and click " Course completion " Expand the " Condition: Activity completion " and tick the activity/resource. Save the changes. Test. Navigate to Course B's course page Open the cog menu and click " Course completion " Expand "Condition: Completion of other courses" Select 'Course A' Save changes Open course completion page again and expand the "Condition: Completion of other courses" section. Confirm that Course A is still selected Deselect Course A (CTRL+left mouse click on Course A for stables, or press the x on Course A for master) Save changes Open course completion page again and expand the "Condition: Completion of other courses" section. Confirm that 'Course A' is no longer selected.