API to create/update user with custom lookup field on user

Hi,

I have a custom field 'doctor' on user object, the doctor refers to another user, it is a lookup field. I'm trying to create/udpate user object through create/update API.

The payload is something like this: 
{

     user: {

       name: 'xxx',

       user_fields: {

              doctor: 13456678 // The doctor id in zendesk.

        }

     }

}

I have problems with this. If I create a user for the first time, I can create the user, but doctor field is empty when I check the Web console. I manually change the doctor to someone in the Web GUI, then call create_or_update API, the doctor will be changed to the one I wanted.

I can also reproduce this issue:  when I manually change the doctor to empty in Web, it will also fail to update the doctor.

Did anybody have this issue before?