diff --git a/index.d.ts b/index.d.ts
index f29eb8c..0fc2333 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -2,12 +2,12 @@ type Action = {
   type: string;
 };
 
-type Reducer<S> = (state: S, action: Action) => S;
+type Reducer<S, A extends Action> = (state: S, action: A) => S;
 
-export default function reduceReducers<S>(
-  initialState: S | null,
-  ...reducers: Reducer<S>[]
-): Reducer<S>;
-export default function reduceReducers<S>(
-  ...reducers: Reducer<S>[]
-): Reducer<S>;
+export default function reduceReducers<S, A extends Action>(
+  initialState: S | null | undefined,
+  ...reducers: Reducer<S, A>[]
+): Reducer<S, A>;
+export default function reduceReducers<S, A extends Action>(
+  ...reducers: Reducer<S, A>[]
+): Reducer<S, A>;