Why is args.xamlFile blank in Silverlight debugging

Question

In the standard error handler for Silverlight apps, there is reference to args.xamlFile:

if (errorType == "ParserError") {
errMsg += "File: " + args.xamlFile + "     \n";
errMsg += "Line: " + args.lineNumber + "     \n";
errMsg += "Position: " + args.charPosition + "     \n";

}

For some reason, lineNumber and charPosition get populated but not xamlFile. Any idea why? I'm trying to figure out an error with multiple children being added to a Panel, but can't figure where it's actually happening!