diff --git a/Contributing.md b/Contributing.md
index 484d438..b5a974a 100644
--- a/Contributing.md
+++ b/Contributing.md
@@ -113,7 +113,7 @@ npx gulp test
 If you wish to add a reference test for your patch, you can place the file (`foo.pdf` in this example) in the `test/pdfs` directory and run the following script from the PDF.js root folder to add it to the suite:
 
 ```
-node test/add_test.js test/pdfs/foo.pdf
+node test/add_test.mjs test/pdfs/foo.pdf
 ```
 
 Note that this script only supports adding `eq` tests for all pages of the PDF file. If you need a different type or page range, you'll need to update the `test/test_manifest.json` file afterwards.