diff --git a/projects/lib/src/types.ts b/projects/lib/src/types.ts
index db1a984a..2bfef376 100644
--- a/projects/lib/src/types.ts
+++ b/projects/lib/src/types.ts
@@ -104,7 +104,7 @@ export abstract class OAuthStorage {
 export class MemoryStorage implements OAuthStorage {
   private data = new Map<string, string>();
 
-  getItem(key: string): string {
+  getItem(key: string): string | undefined {
     return this.data.get(key);
   }