diff --git a/src/transpile.ts b/src/transpile.ts
index 9ab2c898..44ad25f9 100644
--- a/src/transpile.ts
+++ b/src/transpile.ts
@@ -312,7 +312,7 @@ function transpileBundleImpl(context: BuildContext, target: ts.ScriptTarget) {
   try {
     const files = readdirSync(context.buildDir);
     files.forEach((file) => {
-      if (path.extname(file) === 'js' && file.indexOf('polyfills') === -1 && file.indexOf('sw-toolbox') === -1) {
+      if (path.extname(file) === '.js' && file.indexOf('polyfills') === -1 && file.indexOf('sw-toolbox') === -1) {
         const bundlePath = path.join(context.buildDir, file);
         const bundleContent = readFileSync(bundlePath).toString();
         const tsConfig = getTsConfig(context);
diff --git a/src/uglifyjs.ts b/src/uglifyjs.ts
index 64f1e644..2b0cc906 100644
--- a/src/uglifyjs.ts
+++ b/src/uglifyjs.ts
@@ -34,7 +34,7 @@ export function uglifyjsWorker(context: BuildContext, configFile: string): Promi
       const files = readdirSync(context.buildDir);
 
       files.forEach((file) => {
-        if (extname(file) === 'js' && file.indexOf('polyfills') === -1 && file.indexOf('sw-toolbox') === -1) {
+        if (extname(file) === '.js' && file.indexOf('polyfills') === -1 && file.indexOf('sw-toolbox') === -1) {
           const uglifyJsConfig: UglifyJsConfig = fillConfigDefaults(configFile, taskInfo.defaultConfigFile);
           uglifyJsConfig.sourceFile = join(context.buildDir, file);
           uglifyJsConfig.inSourceMap = join(context.buildDir, uglifyJsConfig.inSourceMap);