ArgumentError: Error #2173: Unable to read object in stream ... does not implement flash.utils.IExternalizable

ArgumentError: Error #2173: Unable to read object in stream.  The class does not implement flash.utils.IExternalizable but is aliased to an externalizable class.

This error seems a little backwards to me.

This is caused by a class being sent from the server that implements IExternalizable but is not compiled into your project.

You can fix it by forcing the linker to include your class definition with something like this:

private static var _link:Array = [com.bakedon.YourClass];