diff --git a/index.js b/index.js
index 0d30b0d..3cf773b 100755
--- a/index.js
+++ b/index.js
@@ -134,7 +134,11 @@ class MergeIntoFile {
             const chunk = compilation.addChunk(fileId);
             chunk.id = fileId;
             chunk.ids = [chunk.id];
-            chunk.files.push(newFileNameHashed);
+            if (chunk.files instanceof Set) {
+              chunk.files.add(newFileNameHashed);
+            } else {
+              chunk.files.push(newFileNameHashed);
+            }
           }
         }
         generatedFiles[newFileName] = newFileNameHashed;
diff --git a/index.node6-compatible.js b/index.node6-compatible.js
index efddee7..026b021 100644
--- a/index.node6-compatible.js
+++ b/index.node6-compatible.js
@@ -267,7 +267,12 @@ var MergeIntoFile = /*#__PURE__*/function () {
                         var chunk = compilation.addChunk(fileId);
                         chunk.id = fileId;
                         chunk.ids = [chunk.id];
-                        chunk.files.push(newFileNameHashed);
+
+                        if (chunk.files instanceof Set) {
+                          chunk.files.add(newFileNameHashed);
+                        } else {
+                          chunk.files.push(newFileNameHashed);
+                        }
                       }
                     }