Internal error in geometry decompositon: problem with just 2 elements in model!

Hi Everyone:

I am trying to build a complicated 3-D model with many channels and such but I ran into a problem very early on. I just have one block attached to one hexaheadron. I am unable to form a union. I must be missing something very basic. Could someone help me out? I have attached my mph file.

Thanks!
Sri.