diff --git a/index.d.ts b/index.d.ts
index c1a16080bf4..1cf5a704337 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -90,12 +90,13 @@ export type ReducersMapObject<S = any, A extends Action = Action> = {
  * @returns A reducer function that invokes every reducer inside the passed
  *   object, and builds a state object with the same shape.
  */
-export function combineReducers<S>(
-  reducers: ReducersMapObject<S, any>
-): Reducer<S>
-export function combineReducers<S, A extends Action = AnyAction>(
-  reducers: ReducersMapObject<S, A>
-): Reducer<S, A>
+export function combineReducers<T extends ReducersMapObject<any, any>>(
+  reducers: T
+): Reducer<InferStateType<T>, InferActionTypes<InferReducerTypes<T>>>
+
+type InferActionTypes<R> = R extends Reducer<any, infer A> ? A : AnyAction
+type InferReducerTypes<T> = T extends Record<any, infer R> ? R : Reducer
+type InferStateType<T> = T extends ReducersMapObject<infer S, any> ? S : never
 
 /* store */
 
diff --git a/test/typescript/reducers.ts b/test/typescript/reducers.ts
index 29376714f3f..eb03eaf6d72 100644
--- a/test/typescript/reducers.ts
+++ b/test/typescript/reducers.ts
@@ -42,7 +42,7 @@ function simple() {
   // Combined reducer also accepts any action.
   const combined = combineReducers({ sub: reducer })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
+  let cs = combined(undefined, { type: 'init' })
   cs = combined(cs, { type: 'INCREMENT', count: 10 })
 
   // Combined reducer's state is strictly checked.
@@ -110,17 +110,18 @@ function discriminated() {
   // typings:expect-error
   s = reducer(s, { type: 'SOME_OTHER_TYPE', someField: 'value' })
 
-  // Combined reducer accepts any action by default which allows to include
-  // third-party reducers without the need to add their actions to the union.
-  const combined = combineReducers({ sub: reducer })
+  // Combined reducer accepts a union actions types accepted each reducer,
+  // which can be very permissive for unknown third-party reducers.
+  const combined = combineReducers({
+    sub: reducer,
+    unknown: (state => state) as Reducer
+  })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
-  cs = combined(cs, { type: 'SOME_OTHER_TYPE' })
+  let cs = combined(undefined, { type: 'init' })
+  cs = combined(cs, { type: 'SOME_OTHER_TYPE', someField: 'value' })
 
   // Combined reducer can be made to only accept known actions.
-  const strictCombined = combineReducers<{ sub: State }, MyAction>({
-    sub: reducer
-  })
+  const strictCombined = combineReducers({ sub: reducer })
 
   strictCombined(cs, { type: 'INCREMENT' })
   // typings:expect-error
@@ -179,7 +180,7 @@ function typeGuards() {
 
   const combined = combineReducers({ sub: reducer })
 
-  let cs: { sub: State } = combined(undefined, { type: 'init' })
+  let cs = combined(undefined, { type: 'init' })
   cs = combined(cs, { type: 'INCREMENT', count: 10 })
 }