When this happens we get this AttributeError that doesn't tell us anything

Unhandled DevServerException: CrOS auto-update failed for host <HOST>: 0) RootfsUpdateError: Failed to perform rootfs update: AttributeError("'NoneType' object has no attribute 'encode'",), 1) RootfsUpdateError: Failed to perform rootfs update: AttributeError("'NoneType' object has no attribute 'encode'",), 

Example run:
https://uberchromeos-server38.corp.google.com/new_tko/#tab_id=test_detail_view&object_id=578793025