Bug here.  Function is elsewhere:

trajectory.pyc in openmm_boxes(self, frame)
    858 
    859         params = self.unitcell_parameters[frame]
--> 860         v1, v2, v3 = lengths_and_angles_to_box_vectors(params['a'], params['b'], params['c'],
    861             params['alpha']*np.pi/180, params['beta']*np.pi/180, params['gamma']*np.pi/180)
    862