diff --git a/package.json b/package.json
index a18d2b36..c253ca32 100644
--- a/package.json
+++ b/package.json
@@ -60,7 +60,7 @@
         "c9.ide.language.javascript.tern": "#7aab8b0b6a",
         "c9.ide.language.javascript.infer": "#cfec494a3c",
         "c9.ide.language.jsonalyzer": "#21b64e5820",
-        "c9.ide.collab": "#2f568cc2a7",
+        "c9.ide.collab": "#749c26b859",
         "c9.ide.local": "#2bfd7ff051",
         "c9.ide.find": "#6cc6d3379d",
         "c9.ide.find.infiles": "#72582de3cd",
diff --git a/plugins/c9.ide.ace/ace.js b/plugins/c9.ide.ace/ace.js
index 6459aad0..c7e35fd7 100644
--- a/plugins/c9.ide.ace/ace.js
+++ b/plugins/c9.ide.ace/ace.js
@@ -313,7 +313,10 @@ define(function(require, exports, module) {
                             stack[i + 1].deltas = swapped[1];
                             i++;
                         }
-                        break;
+                        deltaSet = stack[pos - 1].deltas;
+                        if (deltaSet[0].disabled)
+                            deltaSet[0].disabled = false;
+                        return deltaSet;
                     }
                 }
             },
@@ -322,8 +325,10 @@ define(function(require, exports, module) {
                     deltas: deltas,
                     undo: function(){
                         updateDeltas(this.deltas); // change to the new format
-                        _self.rebase();
-                        _self.$session.session.undoChanges(this.deltas, _self.dontSelect);
+                        var deltas = _self.rebase();
+                        if (!deltas)
+                            return false;
+                        _self.$session.session.undoChanges(deltas, _self.dontSelect);
                     },
                     redo: function(){
                         updateDeltas(this.deltas);