diff --git a/lib/build.js b/lib/build.js
index 45ddf608..f49e5d91 100644
--- a/lib/build.js
+++ b/lib/build.js
@@ -31,7 +31,7 @@ function haveBabelrc(functionsDir) {
   );
 }
 
-function webpackConfig(dir, {userWebpackConfig, useBabelrc}) {
+function webpackConfig(dir, {userWebpackConfig, useBabelrc} = {}) {
   var config = conf.load();
   var envConfig = conf.loadContext(config).environment;
   var babelOpts = { cacheDirectory: true };