NAV 2013 Web Services for User Card error

I am trying to use Web Services in NAV to access the User Card page. I have succesfully set up the Web Service and have been able to call it from my application. I can read users, update users and delete users...however, when i try to use the .Create(ref newUser) method I get an error and can't figure out how to create a new user.

Here is the error:

User Name must have a value in User: User Security ID={6A9F7C77-13D6-43CA-A9F9-FDE1498584E7}. It cannot be zero or empty.

It doesn't matter what values I set in the User, including User_Name...I always get this message.

Any advice?