diff --git a/gluon/dal.py b/gluon/dal.py
index 1c2a76fe..baeb624f 100644
--- a/gluon/dal.py
+++ b/gluon/dal.py
@@ -79,7 +79,7 @@ def _default_validators(db, field):
     if (field.notnull or field.unique) and not field_type in excluded_fields:
         requires.insert(0,validators.IS_NOT_EMPTY())
     elif not field.notnull and not field.unique and requires:
-        requires[0] = validators.IS_EMPTY_OR(requires[0])
+        requires[0] = validators.IS_EMPTY_OR(requires[0], null='' if field in ('string', 'text', 'password') else None)
     return requires
 
 from gluon.serializers import custom_json, xml