If a base class includes TinySweeper and defines some swept fields, and you call sweep_up! on its subclass, you get this error:
NoMethodError: undefined method `each' for nil:NilClass
from ...tiny_sweeper-0.0.5/lib/tiny_sweeper.rb:20:in `sweep_up!'
from ...tiny_sweeper-0.0.5/lib/tiny_sweeper.rb:51:in `sweep_up!'