Using Pg's error message

I'm using the Pg package that came with 6.3, and I'm having problems using the
error codes generated by $result->resultStatus. Basically I just want to know
if a query came back null before I try to use it. Here's the code

$result = $conn2->exec("select totalusage from usage where name =
\'$username\'\;");

if ($result->resultStatus == PGRES_EMPTY_QUERY)
{
        print "BAD VOODOO!\n";
}

The error message I continue to receive on empty queries is

PQgetvalue: There is no row 0 in the query results.  The highest numbered row
is -1.

Can anyone tell me what I'm doing wrong here, and point me in the right
direction to using the error codes correctly?

thanks a ton!

greg