diff --git a/lib/Server.js b/lib/Server.js
index 7688da5dfc..6e407a55da 100644
--- a/lib/Server.js
+++ b/lib/Server.js
@@ -490,29 +490,7 @@ class Server {
       options.devMiddleware = {};
     }
 
-    // normalize `headers`
-    if (typeof options.headers !== "undefined") {
-      const allHeaders = {};
-
-      if (Array.isArray(options.headers)) {
-        options.headers.forEach((header) => {
-          allHeaders[header.key] = header.value;
-        });
-        options.headers = allHeaders;
-      }
-
-      if (typeof options.headers === "function") {
-        const returnedHeaders = options.headers();
-
-        if (Array.isArray(returnedHeaders)) {
-          returnedHeaders.forEach((header) => {
-            allHeaders[header.key] = header.value;
-          });
-
-          options.headers = allHeaders;
-        }
-      }
-    }
+    // No need to normalize `headers`
 
     if (typeof options.historyApiFallback === "undefined") {
       options.historyApiFallback = false;
@@ -1867,10 +1845,19 @@ class Server {
         headers = headers(req, res, this.middleware.context);
       }
 
-      // eslint-disable-next-line guard-for-in
-      for (const name in headers) {
-        res.setHeader(name, headers[name]);
+      const allHeaders = [];
+
+      if (typeof headers === "object" && !Array.isArray(headers)) {
+        // eslint-disable-next-line guard-for-in
+        for (const name in headers) {
+          allHeaders.push({ key: name, value: headers[name] });
+        }
+        headers = allHeaders;
       }
+
+      headers.forEach((header) => {
+        res.setHeader(header.key, header.value);
+      });
     }
 
     next();