diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 10ee896..5f974f6 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -298,7 +298,7 @@
     # @param element The element to add.
 
     def append(self, element):
-        # assert iselement(element)
+        self._assert_is_element(element)
         self._children.append(element)
 
     ##
@@ -308,8 +308,8 @@
     # @since 1.3
 
     def extend(self, elements):
-        # for element in elements:
-        #     assert iselement(element)
+        for element in elements:
+            self._assert_is_element(element)
         self._children.extend(elements)
 
     ##
@@ -318,9 +318,13 @@
     # @param index Where to insert the new subelement.
 
     def insert(self, index, element):
-        # assert iselement(element)
+        self._assert_is_element(element)
         self._children.insert(index, element)
 
+    def _assert_is_element(self, e):
+        if not isinstance(e, Element):
+            raise TypeError('expected an Element, not %s' % type(e).__name__)
+
     ##
     # Removes a matching subelement.  Unlike the <b>find</b> methods,
     # this method compares elements based on identity, not on tag