diff --git a/lib/ace/edit_session.js b/lib/ace/edit_session.js
index 544a635c..eb5a8a6f 100644
--- a/lib/ace/edit_session.js
+++ b/lib/ace/edit_session.js
@@ -956,10 +956,12 @@ var EditSession = function(text, mode) {
         var removedFolds = null;
 
         if (action.indexOf("Lines") != -1) {
-            if (action == "insertLines") {
-                lastRow = firstRow + (e.data.lines.length);
-            } else {
-                lastRow = firstRow;
+            if (e.data.lines) {
+                if (action == "insertLines" && e.data.lines) {
+                    lastRow = firstRow + (e.data.lines.length);
+                } else {
+                    lastRow = firstRow;
+                }
             }
             len = e.data.lines ? e.data.lines.length : lastRow - firstRow;
         } else {