Why is it so difficult to load a swf inside another?

programming flash
1

I get all kinds of errors, like:

ReferenceError: Error #1065: Variable flash.geom::Matrix3D is not defined.
	at test998_fla::MainTimeline()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at test998_fla::MainTimeline/initialize()
	at test998_fla::MainTimeline/frame1()

If a swf works by itself shouldn’t it work inside another swf?