diff --git a/src/listener.ts b/src/listener.ts
index 2f266d6..b73acf0 100644
--- a/src/listener.ts
+++ b/src/listener.ts
@@ -67,7 +67,7 @@ const responseViaResponseObject = async (
   outgoing: ServerResponse | Http2ServerResponse,
   options: { errorHandler?: CustomErrorHandler } = {}
 ) => {
-  if (res instanceof Promise) {
+  if (res != null && typeof res.then === 'function') {
     if (options.errorHandler) {
       try {
         res = await res