diff --git a/openerp/addons/base/res/res_users.py b/openerp/addons/base/res/res_users.py
index 47ef13dc1b6..59ff4ecce82 100644
--- a/openerp/addons/base/res/res_users.py
+++ b/openerp/addons/base/res/res_users.py
@@ -32,6 +32,7 @@ from openerp import SUPERUSER_ID, models
 from openerp import tools
 import openerp.exceptions
 from openerp.osv import fields, osv, expression
+from openerp.service.security import check_super
 from openerp.tools.translate import _
 from openerp.http import request
 from openerp.exceptions import UserError
@@ -426,10 +427,7 @@ class res_users(osv.osv):
         return dataobj.browse(cr, uid, data_id, context=context).res_id
 
     def check_super(self, passwd):
-        if passwd == tools.config['admin_passwd']:
-            return True
-        else:
-            raise openerp.exceptions.AccessDenied()
+        return check_super(passwd)
 
     def check_credentials(self, cr, uid, password):
         """ Override this method to plug additional authentication methods"""
diff --git a/openerp/fields.py b/openerp/fields.py
index 360e7938496..84eefe70c97 100644
--- a/openerp/fields.py
+++ b/openerp/fields.py
@@ -1202,13 +1202,15 @@ class Date(Field):
     @staticmethod
     def from_string(value):
         """ Convert an ORM ``value`` into a :class:`date` value. """
+        if not value:
+            return None
         value = value[:DATE_LENGTH]
         return datetime.strptime(value, DATE_FORMAT).date()
 
     @staticmethod
     def to_string(value):
         """ Convert a :class:`date` value into the format expected by the ORM. """
-        return value.strftime(DATE_FORMAT)
+        return value.strftime(DATE_FORMAT) if value else False
 
     def convert_to_cache(self, value, record, validate=True):
         if not value:
@@ -1266,6 +1268,8 @@ class Datetime(Field):
     @staticmethod
     def from_string(value):
         """ Convert an ORM ``value`` into a :class:`datetime` value. """
+        if not value:
+            return None
         value = value[:DATETIME_LENGTH]
         if len(value) == DATE_LENGTH:
             value += " 00:00:00"
@@ -1274,7 +1278,7 @@ class Datetime(Field):
     @staticmethod
     def to_string(value):
         """ Convert a :class:`datetime` value into the format expected by the ORM. """
-        return value.strftime(DATETIME_FORMAT)
+        return value.strftime(DATETIME_FORMAT) if value else False
 
     def convert_to_cache(self, value, record, validate=True):
         if not value:
diff --git a/openerp/models.py b/openerp/models.py
index d16ab4475ba..27e89c4133f 100644
--- a/openerp/models.py
+++ b/openerp/models.py
@@ -1242,7 +1242,8 @@ class BaseModel(object):
         for fun, msg, names in self._constraints:
             try:
                 # validation must be context-independent; call ``fun`` without context
-                valid = not (set(names) & field_names) or fun(self._model, cr, uid, ids)
+                valid = names and not (set(names) & field_names)
+                valid = valid or fun(self._model, cr, uid, ids)
                 extra_error = None
             except Exception, e:
                 _logger.debug('Exception while validating constraint', exc_info=True)
diff --git a/openerp/service/security.py b/openerp/service/security.py
index 72982524ef2..bb719740596 100644
--- a/openerp/service/security.py
+++ b/openerp/service/security.py
@@ -27,10 +27,9 @@ def login(db, login, password):
     return res_users._login(db, login, password)
 
 def check_super(passwd):
-    if passwd == openerp.tools.config['admin_passwd']:
+    if passwd and passwd == openerp.tools.config['admin_passwd']:
         return True
-    else:
-        raise openerp.exceptions.AccessDenied()
+    raise openerp.exceptions.AccessDenied()
 
 def check(db, uid, passwd):
     res_users = openerp.registry(db)['res.users']