diff --git a/_internal/types.ts b/_internal/types.ts
index 95b3cdac0..f2a8b750a 100644
--- a/_internal/types.ts
+++ b/_internal/types.ts
@@ -429,9 +429,9 @@ export type RevalidateCallback = <K extends RevalidateEvent>(
 
 export interface Cache<Data = any> {
   keys(): IterableIterator<string>
-  get(key: Key): State<Data> | undefined
-  set(key: Key, value: State<Data>): void
-  delete(key: Key): void
+  get(key: string): State<Data> | undefined
+  set(key: string, value: State<Data>): void
+  delete(key: string): void
 }
 
 export interface StateDependencies {
diff --git a/_internal/utils/helper.ts b/_internal/utils/helper.ts
index 8d9b8401c..7356b9426 100644
--- a/_internal/utils/helper.ts
+++ b/_internal/utils/helper.ts
@@ -36,7 +36,7 @@ export const createCacheHelper = <Data = any, T = State<Data, any>>(
   const state = SWRGlobalState.get(cache) as GlobalState
   return [
     // Getter
-    () => (cache.get(key) || EMPTY_CACHE) as T,
+    () => ((!isUndefined(key) && cache.get(key)) || EMPTY_CACHE) as T,
     // Setter
     (info: T) => {
       if (!isUndefined(key)) {
@@ -61,7 +61,7 @@ export const createCacheHelper = <Data = any, T = State<Data, any>>(
       }
 
       // If we haven't done any client-side updates, we return the current value.
-      return (cache.get(key) || EMPTY_CACHE) as T
+      return ((!isUndefined(key) && cache.get(key)) || EMPTY_CACHE) as T
     }
   ] as const
 }