diff --git a/index.d.ts b/index.d.ts
index 9399e01d37..4e5016a2cb 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -218,7 +218,7 @@ export interface StoreCreator {
  */
 export type StoreEnhancer<S> = (next: StoreEnhancerStoreCreator<S>) => StoreEnhancerStoreCreator<S>;
 export type GenericStoreEnhancer = <S>(next: StoreEnhancerStoreCreator<S>) => StoreEnhancerStoreCreator<S>;
-export type StoreEnhancerStoreCreator<S> = (reducer: Reducer<S>, preloadedState: S) => Store<S>;
+export type StoreEnhancerStoreCreator<S> = (reducer: Reducer<S>, preloadedState?: S) => Store<S>;
 
 /**
  * Creates a Redux store that holds the state tree.