JavaScript error in Ribbon dropdown

Today I ran in to a very strange issue with SharePoint 2010. All of a sudden, the dropdown menus in the ribbon started to throw javascript error - "null is undefined".

My first guess was that some of our javascript files caused the error (we have quite a lot of code running on the page and in the ribbon). But the error still occured after I disabling all our custom js code.

It later turned out that the error was caused by a empty CSS file! What the h*ll! Ok, fine. You shouldnt load an empty file, but doing so shouldnt break the functionality of the ribbon!

FYI: We loaded the file with CssRegistration.Register("<filename>");