Why are these two strings behaving differently?

[M]

Ok, I am calling a webservice by setting a string to an order id. For
some reason it only works when i hard code the value as a string
literal. No combination of .ToString() String.Copy, String.Concat,
etc works... it only works when the number is hard coded. Any ideas?

query string from browser:
http://localhost/shopfloor/goodsIssue.aspx?oId=1507923

goodsIssue.aspx.cs:
orderId =Request.QueryString["oId"]; //value is set correctly
invokeWebService(string orderId); //string is passed correctly

in web service invocation class:
public ZGetStglocStockZlgortstockRow[] InvokeImpl(string orderId){
ZGetStglocStockRequestBean req = new ZGetStglocStockRequestBean();
ZGetStglocStockZordersRow zOrder = new ZGetStglocStockZordersRow();
zOrder.order = "1507923"; //hard code works
//zOrder.order = orderId; /*this does not work even though
*orderId is 1507923 in the browser,
*and zOrder.order is "1507923" when i
*set a breakpoint and check the value in
VS.
*please help me.
*************************************/
int i = String.Compare(orderId,"1507923"); /*i returns 0...
*this mean they're the
same!!!
*********************/
}