Subclassing a user-defined exception in a user-defined class in Python

Why does this work:

class A:
    class BError(Exception):
        pass
    class CError(AError):
        pass

But this doesn't:

class A:
    class BError(Exception):
        pass
    class CError(A.BError):
        pass

If "method 1" works for calling class variables, why doesn't it work for "class classes"