See traceback and ipdb output at the bottom of this example.

http://nbviewer.ipython.org/github/AASJournals/Tutorials/blob/master/3D_Objects/HYG_Database/HYG.ipynb

I'm at my limit to debug what is actually going on.

But if you ask me why I'm using masked column at all then i'd have to ask you (astropy) why astropy is using masked column for reading non-sparse (non-null containing or flagged) columns in the first place.