[Error when calling login REST API POST method]

I am trying to use the Login REST API Post method and getting the following error:
{"error"

{
    "error": {
        "message": "This value is not allowed",
        "code": 22
    },
    "servertime": 1525220690
}

The URL for the POST action being used with modified login and pass is:

https://iptv.kartina.tv/api/json/login?login=....&pass=....

I have tried using C# code and Postman to issue the API call and both are returning the same failure.
What am I missing?