problems with ibase_free_result (php function)

My setup: PHP 5.0.2, Firebird 1.5

I have some problems with ibase_free_result... It's something like:

Warning: ibase_free_result(): supplied argument is not a valid
Firebird/InterBase result resource in
/opt/lampp/htdocs/klad/include_core/ibase_sessions.php.inc on line 180

the function that is in question look like this:

function ibase_session_destroy($sessionID) {
global $sess_table;
global $db_session;

$query = "DELETE FROM $sess_table WHERE SID = '$sessionID'";
$result = ibase_query($db_session, $query);
ibase_free_result($result);
ibase_commit();
//return(true);
} // end ibase_session_destroy()


$db_session is global variable refering to database connection, it is
opened in function

function ibase_session_open($session_path, $session_name) { //cemu
sluze ova dva argumenta?
global $db_session;
$db_session = ibase_connect(DBHOST, USER, PASS) or die("error in db
connect");
return(true);
} // end ibase_session_open()



I really can't see the problem, starting to think it's some kind of
bug...... Please help!

BTW, these days I worked on a custom firebird/interbase session
handler in php... so if anyone is interested in these functions, I'll
post them here, just ask for it...