Users losing roles

Hi guys. In addition to the usual anonymous and authenticated user roles, I have:
* admin user
* test role
The "admin user" role is old. I just created "test role".

I've discovered that I can't assign the "admin user" role. I edit a user, enable the checkbox beside "admin user", and click the "Save" button. When the request finishes, the "admin user" checkbox is empty, and the "users_roles" database table doesn't have the new row.

However, I'm able to assign "test role" to users.

Any idea what might be going on, and/or how to debug this?

Thanks,
Nick