Solving a Custompress Debugging Error

Hi,

I'm trying to debug my WP install, and have an issue with CustomPress. Any idea why this error is caused?

Notice: Use of undefined constant WPLANG - assumed 'WPLANG' in /Applications/MAMP/htdocs/sxtest/wp-content/plugins/directory/core/custompress/core/core.php on line 57

Thank you for your feedback/help.

Romanzy