BF: supposed fix for the gh-439, but still unable to reproduce OP.

This should help to deal with the issue reported here:

https://github.com/nipy/dipy/issues/439

I am still unable to reproduce the OP, though. @oesteban : would you mind sharing the data that caused the original error to happen? I would like to add that to the tests so that it will not reoccur. Thanks!