diff --git a/lib/ace/edit_session.js b/lib/ace/edit_session.js
index e9e9fadf..a81bf5ed 100644
--- a/lib/ace/edit_session.js
+++ b/lib/ace/edit_session.js
@@ -92,8 +92,10 @@ var EditSession = function(text, mode) {
         doc.on("change", this.onChange.bind(this));
         this.on("changeFold", this.onChangeFold.bind(this));
         
-        this.bgTokenizer.setDocument(this.getDocument());
-        this.bgTokenizer.start(0);
+        if (this.bgTokenizer) {
+            this.bgTokenizer.setDocument(this.getDocument());
+            this.bgTokenizer.start(0);
+        }
     };
 
     this.getDocument = function() {