Request as proxy: Error duplicating a proxy request

Steps to repro:

1. Log into snapshot
2. Go to users and edit a user to add a proxy who can request on their behalf
3. Then go to Inventory, pick an item and create a request
4. In the requester lookup, search for the proxy's name
5. Then, when you see the popup to choose whether to check out as self or on behalf of sponsor, choose sponsor
6. Click save twice to save the record (there is a bug filed for the save button issue here UIREQ-454)
7. Now duplicate the request and choose a requester who isn't a proxy for anyone
8. Click save

Expected: Request should save

Actual: I am unable to save the request as I get a "proxy user ID not valid" error. See attached screencast.