diff --git a/lib/manager/bundler/extract.js b/lib/manager/bundler/extract.js
index 094df6eaee38fd3d3b7b29ecaa7588c1f5128a34..2605811c6d1f3e21f30782ef450ba9be2bc5eb4c 100644
--- a/lib/manager/bundler/extract.js
+++ b/lib/manager/bundler/extract.js
@@ -68,7 +68,7 @@ async function extractPackageFile(content, fileName) {
         lineNumber += 1;
         groupLine = lines[lineNumber];
         if (groupLine !== 'end') {
-          groupContent += groupLine.replace(/^ {2}/, '') + '\n';
+          groupContent += (groupLine || '').replace(/^ {2}/, '') + '\n';
         }
       }
       const groupRes = await extractPackageFile(groupContent);