If user belongs to group sync to Drupal role, non-recoverable error on login or when adding to a group

Adding a user to a civi Group that is synced to a Drupal role leads to a non-recoverable error on login for the user. Error is also triggered for admin (or permissioned users) when adding a user to a group that is synced to a Drupal role.

It *appears* that the user is actually added to the group and the group/role are synced, so it is unclear what the consequences are other than users seeing a disturbing "Unrecoverable Error" message.

Error is not triggered when removing the user from the group.

Traceback and message attached below.