diff --git a/mongoengine/document.py b/mongoengine/document.py
index f1622934..d635d62e 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -1010,6 +1010,7 @@ class DynamicDocument(Document):
         field_name = args[0]
         if field_name in self._dynamic_fields:
             setattr(self, field_name, None)
+            self._dynamic_fields[field_name].null = False
         else:
             super(DynamicDocument, self).__delattr__(*args, **kwargs)