diff --git a/src/karma-webpack.js b/src/karma-webpack.js
index 2124365..33bc1e2 100644
--- a/src/karma-webpack.js
+++ b/src/karma-webpack.js
@@ -58,7 +58,7 @@ function Plugin(
 ) {
   webpackOptions = cloneDeep(webpackOptions);
   webpackMiddlewareOptions = cloneDeep(
-    webpackMiddlewareOptions || webpackServerOptions
+    webpackMiddlewareOptions || webpackServerOptions || {}
   );
 
   const applyOptions = Array.isArray(webpackOptions)