Fatal error: Class 'JTable' not found

[ITN]

So I created just a custom .php file and put line:

Code Select Expand

if (!class_exists( 'VmConfig' )) require(JPATH_ADMINISTRATOR . DS . 'components' . DS . 'com_virtuemart'.DS.'helpers'.DS.'config.php');

However I'm getting this error: "
Fatal error: Class 'JTable' not found in /home/<user>/domains/<domain.com>/public_html/administrator/components/com_virtuemart/helpers/config.php on line 53"

I really need to use some VM classes in that custom .php file.

Any ideas? I tried everything what I found via google.