diff --git a/plone/principalsource/source.py b/plone/principalsource/source.py
index 6ad71db..81feb71 100644
--- a/plone/principalsource/source.py
+++ b/plone/principalsource/source.py
@@ -1,7 +1,11 @@
 from zope.interface import implements
-from z3c.formwidget.query.interfaces import IQuerySource
+from zope.app.component.hooks import getSite
 from zope.schema.interfaces import IContextSourceBinder
+
+from z3c.formwidget.query.interfaces import IQuerySource
+
 from plone.principalsource.term import PrincipalTerm
+
 from Products.CMFCore.utils import getToolByName
 
 class PrincipalSource(object):
@@ -27,7 +31,11 @@ class PrincipalSource(object):
         if not self.users and not self.groups:
             raise ValueError(u"You must enable either users or groups")
         
-        self.acl_users = getToolByName(context, 'acl_users')
+        self.acl_users = getToolByName(context, 'acl_users', None)
+        if self.acl_users is None:
+            site = getSite()
+            if site is not None:
+                self.acl_users = getToolByName(site, 'acl_users', None)
     
     def __contains__(self, value):
         try: