I am a beginner in python and I'm trying to write a function that allows calculate the difference between two fields and save the result in a third. Here is my method:

on_change_ecart def (self, cr, uid, ids, qter, qtep, context = None):
    if context is None:
        context = {}
    return {'value': {'ecartq': qtep - qter}}

and I get the following error:

TypeError: unsupported operand types (s) for -: 'NoneType' and 'unicode'