diff --git a/Core/Commands/DeleteOperatorsCommand.cs b/Core/Commands/DeleteOperatorsCommand.cs
index 0900bf1..c5e2522 100644
--- a/Core/Commands/DeleteOperatorsCommand.cs
+++ b/Core/Commands/DeleteOperatorsCommand.cs
@@ -107,6 +107,11 @@ public void Undo()
 
         public void Do()
         {
+            if (_removeKeyframesCommand == null)
+            {
+                return;
+            }
+
             _removeKeyframesCommand.Do();
             
             var parent = MetaManager.Instance.GetMetaOperator(_parentMetaID);
diff --git a/Core/UndoRedoStack.cs b/Core/UndoRedoStack.cs
index 6785a26..f67ae09 100644
--- a/Core/UndoRedoStack.cs
+++ b/Core/UndoRedoStack.cs
@@ -297,7 +297,7 @@ public bool ProcessReceivedCommands()
 
         public void Add(ICommand command)
         {
-            if (command.IsUndoable)
+            if (command != null && command.IsUndoable)
             {
                 var jsonCommand = SerializeCommand(command);
 #if USE_COUCHDB