The `channel` parameter should take a string instead of integer.  See `channel` parameter of [`Image.composite_channel()`][1] method and [`CHANNELS`][2] constant mapping.

[1]: http://docs.wand-py.org/en/latest/wand/image.html#wand.image.Image.composite_channel
[2]: http://docs.wand-py.org/en/latest/wand/image.html#wand.image.CHANNELS