How can I force use generator on Coffeescript 1.9?

CoffeeScript support generator now, but, I found only you use yield keyword, then your function will be compiled to generator, here is my question, I use koa to write my code, and some middleware I do not need async logic, so I don't need yield, so, CoffeeScript think it is an normal function, but, koa says: app.use() requires a generator function, T^T, anyone have a solution? Thank You!