diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index 658d0c79..348ee977 100644
--- a/mongoengine/base/document.py
+++ b/mongoengine/base/document.py
@@ -147,7 +147,7 @@ class BaseDocument(object):
 
             if not hasattr(self, name) and not name.startswith('_'):
                 DynamicField = _import_class('DynamicField')
-                field = DynamicField(db_field=name)
+                field = DynamicField(db_field=name, null=True)
                 field.name = name
                 self._dynamic_fields[name] = field
                 self._fields_ordered += (name,)
@@ -337,7 +337,7 @@ class BaseDocument(object):
                 value = field.generate()
                 self._data[field_name] = value
 
-            if value is not None:
+            if (value is not None) or (field.null):
                 if use_db_field:
                     data[field.db_field] = value
                 else:
diff --git a/mongoengine/document.py b/mongoengine/document.py
index 71929cf1..c948dac2 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -1013,6 +1013,7 @@ 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)