Expression Cannot Be Evalulated

I have an in C++ and mySQL. It's working mostly, occasional the program breaks on this line.

my_ulonglong numrows = mysql_num_rows(res_set);

It says that it cannot be evalulated. Can anyone shed any light on this? Is it something to do with the records in the database? Any help is appreciated. I've been wrestling three days with this. (mysql_real_connect (conn,"urlock.db.5513143.hostedresource.com","urlock","Admin1234","urlock",0,NULL,0) !=0); char queryString[1024]; sprintf(queryString, "SELECT COUNT(*) FROM tblURLIP WHERE IP = '%s' AND IPStatus = '1' AND IPMax = '0' AND IPType ='3'", ipSrc == 0 ? "0" : ipSrc);

mysql_query(conn, queryString);
开发者_运维技巧my_ulonglong i = 0;
res_set = mysql_store_result(conn);
my_ulonglong numrows = mysql_num_rows(res_set);
LEGIT = mysql_fetch_row(res_set);

This is the error that pops up first. Unhandled exception at 0x57088ce4 in lsniff.exe: 0xC0000005: Access violation reading location 0x00000000

The connection is timing out, when I move to a local database, I don't have the problem.