Issue With WebService in asp.net?

Question

I m working with web services using store procedure and when I m inserted a record using Webservice then inserted successfully but when I click on service description than data not display but the record is inserted successfully in the database and when  I test my web service in postman then get an error see below:

In PostMan:
(post) http://localhost:25880/PaymentMode.asmx?op=login

<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><soap:Fault><soap:Code><soap:Value>soap:Receiver</soap:Value></soap:Code><soap:Reason><soap:Text xml:lang="en">System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.Xml.XmlException: Root element is missing.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at System.Xml.XmlTextReaderImpl.Read()
at System.Web.Services.Protocols.SoapServerProtocol.SoapEnvelopeReader.Read()
at System.Xml.XmlReader.MoveToContent()
at System.Web.Services.Protocols.SoapServerProtocol.SoapEnvelopeReader.MoveToContent()
at System.Web.Services.Protocols.SoapServerProtocolHelper.GetRequestElement()
at System.Web.Services.Protocols.Soap12ServerProtocolHelper.RouteRequest()
at System.Web.Services.Protocols.SoapServerProtocol.RouteRequest(SoapServerMessage message)
at System.Web.Services.Protocols.SoapServerProtocol.Initialize()
at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)
--- End of inner exception stack trace ---</soap:Text></soap:Reason><soap:Detail /></soap:Fault></soap:Body></soap:Envelope>

[WebMethod]
    public string login(string uname, string pass)
    {
        cn.Open();
        SqlCommand cmd = new SqlCommand("loginsp", cn);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@username",uname);
        cmd.Parameters.AddWithValue("@password", pass);
        try
        {
            cmd.ExecuteNonQuery();
            return "inserted data successfully";

        }
        catch (Exception)
        {
            return "inserted data not successfully";
            throw;
        }
        finally
        {
            cn.Close();
        }
    }

store procedure:

CREATE PROCEDURE loginsp
    @username varchar(50),
    @password varchar(50)
AS
BEGIN
    Insert Into studlogin values(@username,@password);
END
GO

problem is that click on service description than data not display but the record is inserted successfully in the database and when I test my web service in postman then get an error??

 

what I m doing wrong in my above code??help??