diff --git a/index.d.ts b/index.d.ts
index c3ea3d04..f3d508ff 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -57,7 +57,7 @@ export interface RefreshResult {
   accessTokenExpirationDate: string;
   additionalParameters?: { [name: string]: string };
   idToken: string;
-  refreshToken: string;
+  refreshToken: string | null;
   tokenType: string;
 }