diff --git a/Lib/test/test_xml_etree_c.py b/Lib/test/test_xml_etree_c.py
index 3a0fc572f457ff..aae8c3b0019951 100644
--- a/Lib/test/test_xml_etree_c.py
+++ b/Lib/test/test_xml_etree_c.py
@@ -234,7 +234,7 @@ class SizeofTest(unittest.TestCase):
     def setUp(self):
         self.elementsize = support.calcobjsize('5P')
         # extra
-        self.extra = struct.calcsize('PnnP4P')
+        self.extra = struct.calcsize('PnnP4PN')
 
     check_sizeof = support.check_sizeof
 
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index e134e096e044b7..3e206aa4318eee 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -230,6 +230,8 @@ typedef struct {
 
     PyObject* _children[STATIC_CHILDREN];
 
+    /* incremented whenever the object is externally mutated */
+    size_t version;
 } ElementObjectExtra;
 
 typedef struct {
@@ -279,6 +281,8 @@ create_extra(ElementObject* self, PyObject* attrib)
     self->extra->allocated = STATIC_CHILDREN;
     self->extra->children = self->extra->_children;
 
+    self->extra->version = 0;
+
     return 0;
 }
 
@@ -506,6 +510,7 @@ element_resize(ElementObject* self, Py_ssize_t extra)
         }
         self->extra->children = children;
         self->extra->allocated = size;
+        self->extra->version++;
     }
 
     return 0;
@@ -539,6 +544,7 @@ element_add_subelement(elementtreestate *st, ElementObject *self,
     self->extra->children[self->extra->length] = Py_NewRef(element);
 
     self->extra->length++;
+    self->extra->version++;
 
     return 0;
 }
@@ -780,6 +786,7 @@ _elementtree_Element___copy___impl(ElementObject *self, PyTypeObject *cls)
 
         assert(!element->extra->length);
         element->extra->length = self->extra->length;
+        element->extra->version = self->extra->version;
     }
 
     return (PyObject*) element;
@@ -847,6 +854,7 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
         if (element_resize(element, self->extra->length) < 0)
             goto error;
 
+        // TODO(picnixz): check for an evil child's __deepcopy__ on 'self'
         for (i = 0; i < self->extra->length; i++) {
             PyObject* child = deepcopy(st, self->extra->children[i], memo);
             if (!child || !Element_Check(st, child)) {
@@ -862,6 +870,7 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
 
         assert(!element->extra->length);
         element->extra->length = self->extra->length;
+        element->extra->version = 0;
     }
 
     /* add object to memo dictionary (so deepcopy won't visit it again) */
@@ -1548,6 +1557,7 @@ _elementtree_Element_insert_impl(ElementObject *self, Py_ssize_t index,
     self->extra->children[index] = Py_NewRef(subelement);
 
     self->extra->length++;
+    self->extra->version++;
 
     Py_RETURN_NONE;
 }
@@ -1645,6 +1655,7 @@ _elementtree_Element_remove_impl(ElementObject *self, PyObject *subelement)
         return NULL;
     }
 
+    // TODO(picnixz): check against evil __eq__
     for (i = 0; i < self->extra->length; i++) {
         if (self->extra->children[i] == subelement)
             break;
@@ -1669,6 +1680,7 @@ _elementtree_Element_remove_impl(ElementObject *self, PyObject *subelement)
     self->extra->length--;
     for (; i < self->extra->length; i++)
         self->extra->children[i] = self->extra->children[i+1];
+    self->extra->version++;
 
     Py_DECREF(found);
     Py_RETURN_NONE;
@@ -1724,6 +1736,7 @@ _elementtree_Element_set_impl(ElementObject *self, PyObject *key,
     if (PyDict_SetItem(attrib, key, value) < 0)
         return NULL;
 
+    self->extra->version++;
     Py_RETURN_NONE;
 }
 
@@ -1757,6 +1770,7 @@ element_setitem(PyObject* self_, Py_ssize_t index, PyObject* item)
             self->extra->children[i] = self->extra->children[i+1];
     }
 
+    self->extra->version++;
     Py_DECREF(old);
 
     return 0;
@@ -1907,6 +1921,7 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
             }
 
             self->extra->length -= slicelen;
+            self->extra->version++;
 
             /* Discard the recycle list with all the deleted sub-elements */
             Py_DECREF(recycle);
@@ -1982,6 +1997,7 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
         }
 
         self->extra->length += newlen - slicelen;
+        self->extra->version++;
 
         Py_DECREF(seq);
 
@@ -2078,6 +2094,7 @@ element_attrib_setter(ElementObject *self, PyObject *value, void *closure)
             return -1;
     }
     Py_XSETREF(self->extra->attrib, Py_NewRef(value));
+    self->extra->version++;
     return 0;
 }