mysql_store_result returns NULL and can't fix it

Im new to c++ and I have been playing with it and mysql, few months ago I did a program that worked fine but now I crashes because mysql_store_result is returning NULL when it shouldn't (If I'm not wrong)

//Executes query
    if (!mysql_query(mysql, checkIpQuery.c_str()))
    {
        if (debug) {
            fprintf(stderr, "%s\n", mysql_error(mysql));
        }
    }

    res = mysql_store_result(mysql);

    int num_fields = mysql_num_fields(res);

    if ((row = mysql_fetch_row(res)))
    { 
              //CODE
            }
        }
    }

When I run this I get this error:

Exception thrown at 0x0FDE1B06 (libmysql.dll) in ArhCore.exe: 0xC0000005: Access violation reading location 0x0000004C.

In this line: int num_fields = mysql_num_fields(res);

I read that I'm getting this error because mysql_store_result is NULL. Any idea of what is happening or what am I missing? Last time I coded was really long time ago and I was still a newbie.