Manager: MultipartDecoder error when selecting rectangle

1. With 0.5.0rc1 from Pip
2. plom-demo
3. Probably irrelevant? I manually ID'd one paper with the client.
4. plom-manager
5. select rectangle
6. boom

Client:

Something unexpected has happened!

Please file a bug and copy-paste the following:

Traceback (most recent call last):
  File "/home/cbm/.local/lib/python3.8/site-packages/plom/manager/manager.py", line 1112, in selectRectangle
    imageList = managerMessenger.IDgetImageFromATest()
  File "/home/cbm/.local/lib/python3.8/site-packages/plom/managerMessenger.py", line 353, in IDgetImageFromATest
    for img in MultipartDecoder.from_response(response).parts:
NameError: name 'MultipartDecoder' is not defined

Doesn't seem to be an error server-side:

Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	RgetProgress GET /REP/progress
Aug18 14:35:43  INFO:routes	IDprogressCount GET /ID/progress
Aug18 14:35:43  INFO:routes	IDgetPredictions GET /ID/predictions
Aug18 14:35:43  INFO:routes	RgetIdentified GET /REP/identified
Aug18 14:35:43  INFO:routes	RgetUserList GET /REP/userList
Aug18 14:37:40  INFO:routes	IDgetImageFromATest GET /ID/randomImage
Aug18 14:37:40  INFO:DB	Sending first unIDd IDPages to manager