Changing user's login fails with "Permission Denied", 303 error

Hello,

I am logging in as an admin (a username we use for the API calls).  Get user's information, they try to update user's login with the following call: 

https://[lithium-server]/[community]/restapi/vc/users/login/[id-of-my-user]/login/set?restapi.session_key=[session_key]

Post Parameters:

value=[new value]

Get this response: 

{"response":{"error":{"message":"Permission Denied","code":303},"status":"error"}} 

Any ideas?