center problems

[spicylemon]

i downloaded the mod thats gives you the option to center your pages, i installed as per instructions then i get this error message when i go to my shop

 

Fatal error: Call to a member function on a non-object in /home/spicyle/public_html/shop/includes/header.php on line 51

 

the code that is on line 51 is

 

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

 

any ideas please