diff --git a/client/src/templates/Challenges/rechallenge/builders.js b/client/src/templates/Challenges/rechallenge/builders.js
index 38ad633ee0..3ea596c579 100644
--- a/client/src/templates/Challenges/rechallenge/builders.js
+++ b/client/src/templates/Challenges/rechallenge/builders.js
@@ -91,7 +91,7 @@ A required file can not have both a src and a link: src = ${src}, link = ${link}
       }
       return '';
     })
-    .reduce((head, element) => head.concat(element), []);
+    .join('\n');
 
   const indexHtml = findIndexHtml(challengeFiles);