hmmm, i cant understand this error

[White_Snake]

yeah its me again o_O

well, now i have worked more on my code and :

Fatal error: Call to a member function on a non-object in /home/whitesnake/public_html/myvbulletintestsite/includes/class_dm_user.php(1414) : eval()'d code on line 2

this is the code of my plugin:
Hook : userdata_presave

PHP Code:

// read the values on user.membergroupids so an array can be created
$membergroupidsarray = $db->query_frist("
                       SELECT membergroupids 
                       FROM " .TABLE_PREFIX. "user
                       WHERE userid = '" . $vbulletin->userinfo['userid'] . "'
                       ");

// chech the value on userfield.field5

$leeuserfield = $db->query_first("
        SELECT field5
        FROM " . TABLE_PREFIX . "userfield
        WHERE userid = '" . $vbulletin->userinfo['userid'] . "'
        ");

// looks if user has choosen the value A on userfield.field5 and loads the created array with a new value on the array containing the value with all the secondary usergroups of the user and if a wasnt choose, it acts for case B

if ($user == 'A' AND $vbulletin->userinfo['posts']== '15')
            {
             $membergroupidsarray[]='9';
             $db->query_frist("
             UPDATE ".TABLE_PREFIX."user
             SET membergroupids=".$membergroupidsarray."
             WHERE userid = '" .$vbulletin->userinfo['userid']. "'
             "); 
            }
if ($user == 'B' AND $vbulletin->userinfo['posts']== '15')
            {
             $membergroupidsarray[]='10';
             $db->query_frist("
             UPDATE ".TABLE_PREFIX."user
             SET membergroupids=".$membergroupidsarray."
             WHERE userid = '" .$vbulletin->userinfo['userid']. "'
             "); 
            } 


help is very apreciated