diff --git a/lib/renderer.js b/lib/renderer.js
index c9077b1..8eb8e88 100644
--- a/lib/renderer.js
+++ b/lib/renderer.js
@@ -68,7 +68,7 @@ default_rules.fence = function (tokens, idx, options, env, slf) {
     if (i < 0) {
       tmpAttrs.push([ 'class', options.langPrefix + langName ]);
     } else {
-      tmpAttrs[i] += ' ' + options.langPrefix + langName;
+      tmpAttrs[i][1] += ' ' + options.langPrefix + langName;
     }
 
     // Fake token just to render attributes