Fejl i tagwall

<?
    print "<b>Tidligere beskeder:</b><br><br>";
    $res = mysql_query("select * from guestbook order by dato desc limit 10;");
    while($gb = mysql_fetch_assoc($res)) {
        $gb[besked] = preg_replace("/</","&lt;",$gb[besked]);
        $gb[besked] = preg_replace("/>/","&gt;",$gb[besked]);
        $gb[besked] = nl2br($gb[besked]);
        $dato = date("F j, H:i",$gb[dato]+(3600*5));
        print "Navn: $gb[navn]<br>
        Dato: $dato<br>
        Besked: $gb[besked]";

        if ($_GET[action] == "admin") {
            print "<br><a href=guestbook.php?action=delete&beskedid=$gb[beskedid]>Slet</a>";
        }

        print "<br><br><br>";
    }
    ?>


Det er min PHP kode.
Og fejlen er:
Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in e:\users\new\superegern\html\guestbook.php on line 34

Linje 34 er: while($gb = mysql_fetch_assoc($res)) {

Nogle der kan hjælpe?