diff --git a/src/create-reducer.ts b/src/create-reducer.ts
index ffb82c0..e38be0b 100644
--- a/src/create-reducer.ts
+++ b/src/create-reducer.ts
@@ -4,6 +4,7 @@ import {
   HandlerMap,
 } from './create-handler-map'
 import { merge } from './utils'
+import { DeepImmutable } from './types'
 
 /**
  * Reducer factory
@@ -21,7 +22,7 @@ export function createReducer<State, HM extends HandlerMap<State, any>>(
   const handlerMap = merge(...handlerMapsCreator(createHandlerMap))
 
   return (
-    state = defaultState,
+    state = <DeepImmutable<State>>defaultState,
     action: HM extends HandlerMap<State, infer T> ? T : never
   ) => {
     const handler = handlerMap[action.type]
diff --git a/src/types.ts b/src/types.ts
index 52e8440..c2ae579 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,4 +1,27 @@
+type Primitive = undefined | null | boolean | string | number | Function
+
+interface DeepImmutableArray<T> extends ReadonlyArray<DeepImmutable<T>> {}
+interface DeepImmutableMap<K, V>
+  extends ReadonlyMap<DeepImmutable<K>, DeepImmutable<V>> {}
+type DeepImmutableObject<T> = { readonly [K in keyof T]: DeepImmutable<T[K]> }
+
+export type Immutable<T> = T extends Primitive
+  ? T
+  : T extends Array<infer U>
+  ? ReadonlyArray<U>
+  : T extends Map<infer K, infer V>
+  ? ReadonlyMap<K, V>
+  : Readonly<T>
+
+export type DeepImmutable<T> = T extends Primitive
+  ? T
+  : T extends Array<infer U>
+  ? DeepImmutableArray<U>
+  : T extends Map<infer K, infer V>
+  ? DeepImmutableMap<K, V>
+  : DeepImmutableObject<T>
+
 export type Reducer<State, Actions> = (
-  prevState: State,
+  prevState: DeepImmutable<State>,
   action: Actions
-) => State
+) => DeepImmutable<State> | State