diff --git a/Foundation/CPUndoManager.j b/Foundation/CPUndoManager.j
index dea9c74a4..7f0b58a83 100644
--- a/Foundation/CPUndoManager.j
+++ b/Foundation/CPUndoManager.j
@@ -729,7 +729,7 @@ if (_currentGroup == nil)
     // Don't add no-ops to the undo stack.
     var before = [aChange valueForKey:CPKeyValueChangeOldKey],
         after = [aChange valueForKey:CPKeyValueChangeNewKey];
-    if (before === after || (after !== nil && after.isa && [before isEqual:after]))
+    if (before === after || (before !== nil && before.isa && (after === nil || after.isa) && [before isEqual:after]))
         return;
 
     [[self prepareWithInvocationTarget:anObject]