diff --git a/index.d.ts b/index.d.ts
index f29eb8c..39dc6d0 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -2,12 +2,26 @@ type Action = {
   type: string;
 };
 
-type Reducer<S> = (state: S, action: Action) => S;
+type Reducer<S> = (state: S | undefined, action: Action) => S;
+type PreloadedReducer<S> = (state: S, action: Action) => S;
 
 export default function reduceReducers<S>(
-  initialState: S | null,
-  ...reducers: Reducer<S>[]
+  initialState: S,
+  ...reducers: PreloadedReducer<S>[]
+): Reducer<S>;
+export default function reduceReducers<S>(
+  initialReducer: Reducer<S>,
+  ...reducers: PreloadedReducer<S>[]
 ): Reducer<S>;
 export default function reduceReducers<S>(
-  ...reducers: Reducer<S>[]
+  initialState: S | null,
+  initialReducer: Reducer<S>,
+  ...reducers: PreloadedReducer<S>[]
 ): Reducer<S>;
+export default function reduceReducers<S>(
+  initialState: S | null,
+  ...reducers: PreloadedReducer<S>[]
+): PreloadedReducer<S>;
+export default function reduceReducers<S>(
+  ...reducers: PreloadedReducer<S>[]
+): PreloadedReducer<S>;