diff --git a/lib/webpack/server.config.js b/lib/webpack/server.config.js
index b58b80dcfe..e1ba11ccc5 100644
--- a/lib/webpack/server.config.js
+++ b/lib/webpack/server.config.js
@@ -63,7 +63,7 @@ export default function () {
 
   // Extend config
   if (typeof this.options.build.extend === 'function') {
-    this.options.build.extend(config, {
+    this.options.build.extend.call(this, config, {
       dev: this.dev,
       isServer: true
     })