Arguments don't work for stubs in mocks

The following doesn't work:

class Foo
  def foo(a)
    a
  end
end

Spectator.describe "mocks" do
  mock Foo do
    stub foo(a) { puts a; a }
  end
end

Results in: Error: undefined local variable or method 'a' for Foo.class

Works fine for doubles, but should also work for mocks.