post json string to restful wcf - The remote server returned an error: (400) Bad Request

Question

I have created restful wcf as :

<OperationContract()> _
    <WebInvoke(Method:="POST", UriTemplate:="/getlatestbill", BodyStyle:=WebMessageBodyStyle.Bare, RequestFormat:=WebMessageFormat.Json, ResponseFormat:=WebMessageFormat.Json)> _
    Function getlatestbill(ByVal usrinfo As String) As String

I have seen many links of POST Restful WCF with ContentXml or a class, but i want to pass JsonString. In above code am passing userinfo as json String which contains id and city. as

Sub Post()
        Dim req As HttpWebRequest = Nothing
        Dim res As HttpWebResponse = Nothing
        Try
            Dim strg As String
            strg = New JavaScriptSerializer().Serialize(New With {Key .ID = 1, Key .city = "dehradun"})
            Const url As String = "http://localhost:1843/mobsrv.svc/getlatestbill"
            req = CType(WebRequest.Create(url), HttpWebRequest)
            req.Method = "POST"
            req.ContentType = "application/json; charset=utf-8"
            req.Timeout = 30000
            'req.Headers.Add("SOAPAction", url)

            req.ContentLength = strg.Length
            Dim sw = New StreamWriter(req.GetRequestStream())
            sw.Write(strg)
            sw.Close()

            res = CType(req.GetResponse(), HttpWebResponse)
            Dim responseStream As Stream = res.GetResponseStream()
            Dim streamReader = New StreamReader(responseStream)

         Catch ex As Exception
            Response.Write(ex.Message)
        End Try
    End Sub

while passing the json string to the service its throwing error in res.GetResponseStream() as :

The remote server returned an error: (400) Bad Request.

I have seen many SO posts but none of them accept jsonString in post function.

How to pass json String via POST to wcf restful service?