Object invalid during intersect attempt

Hello-

I'm trying to create a pattern.
I drew a rose with the artistic media tool, grouped it, 'coloured' it in by placing an object created from the rectangle tool behind it, and grouped that to make what I thought was one object. When I try intersect the rose with a new rectangle object I get 'Nothing Changed'. If I select the new rectangle object first, then the rose, I get 'The object you selected is invalid.' I discovered the 'colour' object in the rose group can be intersected, it's the group of objects from the 'line drawing' via the artistic media tool that won't intersect.

If that makes any sense, could someone please tell me how to validate my rose? :)

Thanks!

Mulberi