Exporting geometry to .STL

Hi,

I have a 3D geometry that i wanted to export as a .STL file. But everytime 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. Any help on how i could export it? 

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

Thanks,

Abhay.