Red error message appearing in Custom Fields editor

In the bugfest-iris environment (login folio / folio)
When I am in Settings>Users>custom fields and I click edit
I expect to see the edit view of all the custom fields.

Actual results:
I DO see the edit view, bit also the error message pasted below.

However, you can continue to create, edit or delete custom fields. Sometimes you get the message upon saving, but it doesn't affect the functionality.