Need help with libpq and PQexec

Hi,
In my source code, I have:

res = PQexec(conn,"select * from resource");
if(PQresultStatus(res) != PGRES_COMMAND_OK){
            switch_log_printf(SWITCH_CHANNEL_LOG,SWITCH_LOG_ERROR,"PQexec failed:%s\n",PQerrorMessage(conn));
            PQclear(res);
            return NULL;
        }
        PQclear(res);

When it is executed, I am getting PGexec failed error , but the return value of  PQerrorMessage(conn) is empty.

Does anyone know why?

Thanks,
JB