AutoCAD 2013 :: Exporting Geometry To STL

I have a 3D geometry that i wanted to export as a .STL file. But every time i selected the geometry to export, i keep getting the option "Select solids or watertight meshes: Specify opposite corner:." Although the command line does show that the objects have been selected. I've tried different ways of selecting the geometry but every time it shows the same thing.

P.S - i was able to export it in other (.IGES, .IGS formats) but it doesn't seem to be working the same way with .STL