Cura fills holes

[Johnen 0]

I'm pretty new to 3d-printing and I was just doing a model in sketchup for a spare part, and printing it with my friends 3d-printer.

But Cura says its manifold. And when I slice it, Cura fills the empty spaces up.

 

I cant see whats wrong with the model... Anyone that can give any suggestions?

Adapter.stl

[GregValiant 1,119]

When I load it I get "Not watertight" and it's made up of 4 meshes.  Try looking at it in X-Ray view.

I don't think it exported correctly from SketchUp.