diff --git a/lib/toc.js b/lib/toc.js
index 378259f..ef73f45 100644
--- a/lib/toc.js
+++ b/lib/toc.js
@@ -61,7 +61,11 @@ $.fn.toc = function(options) {
 
       //add anchor
       if(heading.id !== anchorName) {
-        var anchor = $('<span/>').attr('id', anchorName).insertBefore($h);
+        if (!heading.id) {
+          heading.id = anchorName;
+        } else {
+          var anchor = $('<span/>').attr('id', anchorName).insertBefore($h);
+        }
       }
 
       //build TOC item