Installing a theme, having a problem

[belgin fish]

hey, well i got a theme and it says after you've installed oscommerce to then install the theme, and when i go to the theme to install it i get

 

Fatal error: Call to a member function size() on a non-object in /home/templeof/public_html/onetopstore.com/shop/install/templates/main_page/header.php on line 51

 

Any ideas?

 

Line 51 : if ($messageStack->size('header') > 0) {

 

 

Around line 51 :

 

 

if ($messageStack->size('header') > 0) {

echo $messageStack->output('header');

}