Can't change the base class

I have a library symbol in a Flash project that uses a base class in Actionscript. I wrote a new Actionscript class, which is saved in the same directory as the old one and inherits from the same superclass (so it's definitely a subclass of MovieClip). However, when I go to the Properties window and try to change the Base Class field, it won't let me save the change to the new class; instead I get an error message that says "A definition for the base class could not be found in the classpath. Please enter the name of a class that is defined in the classpath, or enter the default base class 'flash.display.MovieClip'. What am I doing wrong? Why doesn't Flash want to admit that this class exists?