diff --git a/lib/ace/placeholder_test.js b/lib/ace/placeholder_test.js
index fb1ddf01..bafb1de3 100644
--- a/lib/ace/placeholder_test.js
+++ b/lib/ace/placeholder_test.js
@@ -140,6 +140,8 @@ module.exports = {
     "test: cancel": function(next) {
         var session = new EditSession("var a = 10;\nconsole.log(a, a);", new JavaScriptMode());
         session.setUndoManager(new UndoManager());
+        // hack around the difference between full ACE context and C9 world
+        session.getUndoManager().$undostack = session.getUndoManager().$undoStack;
         var editor = new Editor(new MockRenderer(), session);
         var p = new PlaceHolder(session, 1, {row: 0, column: 4}, [{row: 1, column: 12}, {row: 1, column: 15}]);