AgChoice: call to ValidateUser and the server response

gistfile1.json

This is the call:

{
    "method": "ValidateUser",
    "params": [
        {
            "userName": "some-user",
            "password": "some-pass"
        }
    ],
    "id": 1
}

This is the response:

{
    "id": null,
    "error": {
        "name": "JSONRPCError",
        "message": "Cannot import System.String from a JSON Object value.",
        "errors": [
            {
                "name": "JsonException",
                "message": "Cannot import System.String from a JSON Object value."
            }
        ]
    }
}

I tried these URLs:

http://agchoice-dev2.tradeslot.com/ios.ashx
http://agchoice-dev2.tradeslot.com/ios.ashx?test