Non manifold?

I'm testing out the CoCreate program, which is pretty much my first 3D CAD program other that Google Sketchup. While it does pretty much everything i want it to do, I'm still getting 'non manifold' issues, even with a simple cylinder.

I ran the in built checks, and not a single error turned up. What am i doing wrong?

EDIT:, Oh, and i forgot to mention that i did use the error finding applet on shapeways, but when it loade nothing appeared. Just a blank screen (well, except the background ).

Sorry, everyone. It seems i accidentally missed the advice on using MinMagics. Now that I've run the file through MiniMagics, it's perfectly fine to print.

Again, sorry.