Kotlin - How to create an alias function of RxJava flatmap()?

I tried to create an alias function to Flowable.flatmap() as follow, but compile error.

fun <T, R> Flowable<T>.then(mapper: Function<T, Publisher<R>>): Flowable<R> {
  return flatMap(mapper)
}

The error is : One type argument expected for interface Function<out R> defined in kotlin

Have any idea? Thanks!