diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index f31c22ce..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,)