Custom Base Classes and Custom Classes

I’ve created a base class to house some basic functions I’d like to have across several movie clips in a game I’m making, however, when I link a custom class to the same movie clip I get the message;

“The base class will not be used because the class specified is already defined and extends its own subclass. If you wish to use the base class, please specify a class name in the Class field that will be auto-generated or enter the default base class ‘flash.display.MovieClip’ in the Base Class field.”

I removed the ‘extends MovieClip’ inside the Class but it still shows the message.

Does this mean that I cannot use a custom class and a custom base class on a movie clip?