diff --git a/docs/middleware.md b/docs/middleware.md
index 327f61179675e..a660afd0f81c3 100644
--- a/docs/middleware.md
+++ b/docs/middleware.md
@@ -76,6 +76,18 @@ Middleware runs directly after `redirects` and `headers`, before the first files
 
 Middleware uses a [strict runtime](/docs/api-reference/edge-runtime.md) that supports standard Web APIs like `fetch`. This works out of the box using `next start`, as well as on Edge platforms like Vercel, which use [Edge Functions](http://www.vercel.com/edge).
 
+## Custom Server
+
+When using a custom server with middleware, you must specify the hostname and port when instantiating your `NextApp`.
+
+```ts
+import next from 'next'
+// ...
+const port = process.env.PORT ? +process.env.PORT : 3000
+const dev = process.env.NODE_ENV !== 'production'
+const app = next({ dev, customServer: true, hostname: 'localhost', port })
+```
+
 ## Related
 
 <div class="card">