diff --git a/index.d.ts b/index.d.ts
index fdc8465..dc6704b 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -18,14 +18,24 @@ declare module 'fastify' {
 }
 
 export interface FastifyBasicAuthOptions {
-  validate(
-    this: FastifyInstance,
-    username: string,
-    password: string,
-    req: FastifyRequest,
-    reply: FastifyReply,
-    done: (err?: Error) => void
-  ): void | Promise<void|Error>;
+  validate: (
+    (
+      this: FastifyInstance,
+      username: string,
+      password: string,
+      req: FastifyRequest,
+      reply: FastifyReply,
+    ) => Promise<Error | void>
+  ) | (
+    (
+      this: FastifyInstance,
+      username: string,
+      password: string,
+      req: FastifyRequest,
+      reply: FastifyReply,
+      done: (err?: Error) => void
+    ) => void
+  );
   authenticate?: boolean | { realm: string | ((req: FastifyRequest) => string) };
   header?: string;
 }
diff --git a/index.test-d.ts b/index.test-d.ts
index e24a56c..efd46ad 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -19,6 +19,7 @@ app.register(fastifyBasicAuth, {
     expectType<FastifyRequest>(req)
     expectType<FastifyReply>(reply)
     expectType<FastifyInstance>(this)
+    if (Math.random() > 0.5) return new Error()
   },
   header: 'x-forwarded-authorization'
 })