diff --git a/src/types.ts b/src/types.ts
index 9a3265f..cec9560 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,6 +1,6 @@
 import { Sink, Stream } from '@most/types'
 
-export type Subject<A, B> = [Sink<A>, Stream<B>]
+export type Subject<A, B> = [AttachSink<A>, Stream<B>]
 
 export interface AttachSink<A> extends Sink<A> {
   attach(source: Stream<A>): Stream<A>