diff --git a/types/types.d.ts b/types/types.d.ts
index d1eed51..91d199b 100644
--- a/types/types.d.ts
+++ b/types/types.d.ts
@@ -26,7 +26,6 @@ type FastifySession = FastifyPluginCallback<fastifySession.FastifySessionOptions
 }
 
 type Callback = (err?: Error) => void;
-type CallbackSession = (err: Error | null, result: Fastify.Session) => void;
 
 interface SessionData extends ExpressSessionData {
   sessionId: string;
@@ -80,6 +79,8 @@ interface Signer {
 }
 
 declare namespace fastifySession {
+  export type CallbackSession = ((err: Error) => void) | ((err: null, result: Fastify.Session) => void)
+
   export interface SessionStore {
     set(
       sessionId: string,
diff --git a/types/types.test-d.ts b/types/types.test-d.ts
index 7643f7f..20d26eb 100644
--- a/types/types.test-d.ts
+++ b/types/types.test-d.ts
@@ -88,8 +88,10 @@ app.route({
     request.sessionStore.set('session-set-test', request.session, () => {});
     request.sessionStore.get('', (err, session) => {
       expectType<Error | null>(err);
-      expectType<Session>(session);
-      expectType<{ id: number } | undefined>(session.user);
+      expectType<Session | undefined>(session);
+      if (session) {
+        expectType<{ id: number } | undefined>(session.user);
+      }
     });
     expectType<void>(request.session.set('foo', 'bar'));
     expectType<string>(request.session.get('foo'));