diff --git a/index.d.ts b/index.d.ts
index 91e85bb..422f9d3 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -10,8 +10,8 @@ declare namespace JwksRsa {
 
     getKeys(): Promise<unknown>;
     getSigningKeys(): Promise<SigningKey[]>;
-    getSigningKey(kid: string): Promise<SigningKey>;
-    getSigningKey(kid: string, cb: (err: Error | null, key: SigningKey) => void): void;
+    getSigningKey(kid?: string | null | undefined): Promise<SigningKey>;
+    getSigningKey(kid: string | null | undefined, cb: (err: Error | null, key: SigningKey) => void): void;
   }
 
   interface Headers {