Python 3.5 invalid signature | Pepper robot

I'm using Python 3.5 with the Pepper robot from Softbank. I want to subscribe to events with

pepper = qi.Application()
pepper.start()
session = pepper.session
memory = session.service("ALMemory")

subscriber = memory.subscriber("ALTouch/TouchChanged")
    if subscriber:
        subscriber.signal.connect(touch_event)

The fifth line throws an RuntimeError: Invalid signature.

Does anyone know how to make this work?