[flexcoders] Wierd error 1067 on extended DefaultDataDescriptor

[Amy]

I have a class, LazyLoadingDataDescriptor, which extends 
DefaultDataDescriptor.  I'm getting a compiler error 1067: Implicit 
coercion of a value of type Class to an unrelated type 
mx.controls.treeClasses:ITreeDataDescriptor.

Since this is extending a class that implements ITreeDataDescriptor2, 
shouldn't I be able to set it as the DataDescriptor of a tree?

TIA;

Amy