diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index f8dd8400..12d3dfa0 100644
--- a/mongoengine/base/document.py
+++ b/mongoengine/base/document.py
@@ -491,7 +491,7 @@ class BaseDocument(object):
                 # remove lower level changed fields
                 level = '.'.join(levels[:idx]) + '.'
                 remove = self._changed_fields.remove
-                for field in self._changed_fields:
+                for field in self._changed_fields[:]:
                     if field.startswith(level):
                         remove(field)