supplied argument is not a valid MySQL result resource

Når jeg skal prøve at aktivere en brugerkonto med det her script

<?
/* Account activation script */

// Get database connection
include 'db.php';

// Create variables from URL.

$userid = $_REQUEST['id'];
$code = $_REQUEST['code'];

$sql = mysql_query("UPDATE user SET activated='1' WHERE userID='$userid' AND password='$code'");

$sql_doublecheck = mysql_query("SELECT * FROM user WHERE userID='$userid' AND password='$code' AND activated='1'");
$doublecheck = mysql_num_rows($sql_doublecheck);

if($doublecheck == 0){
    echo "<strong><font color=red>Deres Konto kunne ikke blive aktiveret!</font></strong>";
} elseif ($doublecheck > 0) {
    echo "<strong>Deres konto er nu aktiveret!</strong> You may login below!<br />";
    include 'login_form_skigol.html';
}

?>

får jeg denne her fejl: 


Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /hsphere/local/home/skigol/skigol.dk/skigoluser/activateskigol.php on line 15
Deres Konto kunne ikke blive aktiveret!