Problem with "mysqlpp:Result"

[ALeX]

I have this little problem, if i use this code of exemple:
if (res) {
		mysqlpp::Row row;
		mysqlpp::Row::size_type i;
		for (i = 0; row = res.at(i); ++i) {
			cout << '\t' << row.at(0) << endl;
		}
	}
My executable stop with "This application has requested the Runtime to
terminate it in an unusual way.Please contact the application's
support team for more information."

I solve with this trick...
if (res) {
   mysqlpp::Row row;
   mysqlpp::Row::size_type i;
   for (i = 0; i < res.size(); ++i){
       row = res.at(i);
  out << '\t' << row.at(0) << endl;
   }

My question is why the first code rise errors?