problem:concatenating string

[Awais Abbasi]

Hi all
I am trying to concatenate strings but XmlRpcValue is throwing error
this is my code for concatenation
I dont know why its throwing error or whether XmlRpcValue  should be throwing any error

std::string resultString  = "";
for (int i=0; i < 50; i++)
{
        resultString += "<Field>";
        resultString += (std::string)params[i]["Field"];
        resultString += "</Field>";
}

concatenation happens if I do
resultString += "<Field>";
resultString += "</Field>";

but as soon as i try to concatenate
(std::string)params[i]["Field"];
error is thrown

I have checked
(std::string)params[i]["Field"]; as it does return a string

Your help  is needed here
Thanks

[Awais Abbasi]

sorry error is not in XmlRpcValue
but in

XmlRpcServerConnection::executeRequest:fault type error.