diff --git a/src/index.ts b/src/index.ts
index 0ade383..4520d7a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,12 +2,12 @@ export interface Transform<A, B> {
   (source: Source<A>): Source<B>
 }
 
-export type Source<T> = AsyncIterable<T> | Iterable<T>
-
-export interface Sink<T, R = Promise<void>> {
+export interface Sink<T, R> {
   (source: Source<T>): R
 }
 
+export type Source<T> = AsyncIterable<T> | Iterable<T>
+
 export interface Duplex<TSource, TSink = TSource, RSink = Promise<void>> {
   source: Source<TSource>
   sink: Sink<TSink, RSink>