diff --git a/src/index.js b/src/index.js
index d65b335..942d786 100644
--- a/src/index.js
+++ b/src/index.js
@@ -103,8 +103,12 @@ class WebpackRTLPlugin {
           }
         })
 
-        cssnanoPromise.then(() => {
-          chunk.files.push.apply(chunk.files, rtlFiles)
+       cssnanoPromise.then(() => {
+          if (chunk.files instanceof Set) {
+            rtlFiles.forEach( item => chunk.files.add( item ) );
+          } else {
+            chunk.files.push.apply(chunk.files, rtlFiles)
+          }
           cb()
         })
       }, callback)