Python: message_filter AttributeError when registering callback

edit

So I've wrapped my python node in a class, and I've been trying to approximate sync two messages at once (one being Image and another custom message ofMsg that consists of two Float32s) as below:

Class Node:
    def __init__(self):
        self.image_sub = rospy.Subscriber("/usb_cam/image_raw", Image)
        self.opticalflow_sub = rospy.Subscriber("/opticalflow", ofMsg)
        self.ts = message_filters.ApproximateTimeSynchronizer([self.image_sub, self.opticalflow_sub], 10, 0.1, allow_headerless=True)
        self.ts.registerCallback(self.callback)

    def callback(self, image, custom_message):
        #...

    def main(self):
        rospy.init_node('node')
        while not rospy.is_shutdown():
            rospy.spin()
            self.rate.sleep()

However, I get: AttributeError: 'Subscriber' object has no attribute 'registerCallback'

I've been looking at other questions regarding message_filters in python with approximate time synchronizes, and they seem to have implemented things similarly (albeit outside of a class) and I can't really see where I might have gone wrong. My package.xml seems to check out as well, catkin_make works just fine.

Any suggestions?

(I'm using Ubuntu 16.04 on kinetic using python2.7)