fix none error [ci skip]
fix none error [ci skip]

jupyter/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader