Can't re-assign a case to another contact

I managed to reproduce this bug on 4.6.2 demo and local:

• create a new Case
• go to the "roles" section, and click on the "edit" icon to re-assign to someone else.
• assign to another individual, click save

Result: the roles snippet refreshes, but has the same person assigned. Looking at the network request, it fails with a fatal error.

If I delete the person from the case, and add again, it works fine.