diff --git a/Products/CMFPlone/Portal.py b/Products/CMFPlone/Portal.py
index cbf4c91149..878872e666 100644
--- a/Products/CMFPlone/Portal.py
+++ b/Products/CMFPlone/Portal.py
@@ -61,7 +61,7 @@ def __getattr__(self, name):
 
     def __setattr__(self, name, obj):
         # handle re setting an item as an attribute
-        if self._tree is not None and name in self:
+        if not name.startswith("_") and self._tree is not None and name in self:
             del self[name]
             self[name] = obj
         else:
diff --git a/news/4026.bugfix b/news/4026.bugfix
new file mode 100644
index 0000000000..a51ab29b2e
--- /dev/null
+++ b/news/4026.bugfix
@@ -0,0 +1 @@
+Avoid POSKeyError when commit occurs and we have savepoint that involves Plone Site. @wesleybl