diff --git a/lib/ace/background_tokenizer.js b/lib/ace/background_tokenizer.js
index 0ee8990c..e5fe73bd 100644
--- a/lib/ace/background_tokenizer.js
+++ b/lib/ace/background_tokenizer.js
@@ -130,8 +130,8 @@ var BackgroundTokenizer = function(tokenizer, editor) {
     };
 
     this.$tokenizeRows = function(firstRow, lastRow) {
-        if (!this.doc)
-            return [];
+        if (!this.doc || isNaN(firstRow) || isNaN(lastRow))
+            return [{'state':'start','tokens':[]}];
             
         var rows = [];