diff --git a/lib/execjs/external_runtime.rb b/lib/execjs/external_runtime.rb
index 164587c..299f696 100644
--- a/lib/execjs/external_runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -59,7 +59,7 @@ def create_tempfile(basename)
         end
 
         def write_to_tempfile(contents)
-          tmpfile = create_tempfile(['execjs', 'js'])
+          tmpfile = create_tempfile(['execjs', '.cjs'])
           tmpfile.write(contents)
           tmpfile.close
           tmpfile