diff --git a/types/smtp-server/index.d.ts b/types/smtp-server/index.d.ts
index 4d670c100b..41306dc9f6 100644
--- a/types/smtp-server/index.d.ts
+++ b/types/smtp-server/index.d.ts
@@ -306,7 +306,7 @@ export class SMTPServer extends EventEmitter {
     listen(handle: any, listeningListener?: () => void): net.Server; // tslint:disable-line unified-signatures
 
     /** Closes the server */
-    close(callback: () => void): void;
+    close(callback?: () => void): void;
 
     updateSecureContext(options: tls.TlsOptions): void;