PATCH / subscriptions / {sid} returns "InternalServerError"

In Azure API Management REST API Subscription entity when I call

PATCH /subscriptions/xxxxxxxxxxxxxxxx?api-version=2014-02-14-preview

with request body containing primaryKey=yyyyyyyyyyy

, it returns unexpected 500 Internal Server Error

with body:

{
    "error": {
        "code": "InternalServerError",
        "message": "Request processing failed due to internal error.",
        "details": null
    }
}

      

        
        
        
      

    

Am I doing something wrong?

Documentation: http://msdn.microsoft.com/en-us/library/azure/dn776325.aspx#UpdateSubscription