Fix warning in overwritegradecells
Fix warning in overwritegradecells

ellisonbg/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,alope107/nbgrader,jhamrick/nbgrader,MatKallada/nbgrader,EdwardJKim/nbgrader,MatKallada/nbgrader,jupyter/nbgrader,modulexcite/nbgrader,modulexcite/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,dementrock/nbgrader,ellisonbg/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,jdfreder/nbgrader,jupyter/nbgrader,alope107/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jdfreder/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader