diff --git a/src/collective/z3cform/rolefield/field.py b/src/collective/z3cform/rolefield/field.py
index 70856aa..9981130 100644
--- a/src/collective/z3cform/rolefield/field.py
+++ b/src/collective/z3cform/rolefield/field.py
@@ -1,9 +1,9 @@
 # -*- coding: utf-8 -*-
-from zope.component import adapts
+from zope.component import adapts, getUtility
 from zope.interface import implementer
 from zope.interface import Interface
 
-from zope.schema.interfaces import IList
+from zope.schema.interfaces import IList, IVocabularyFactory
 from zope.schema import List, Tuple
 from zope.interface import Invalid
 
@@ -61,7 +61,7 @@ class LocalRolesToPrincipalsDataManager(AttributeField):
         """See z3c.form.interfaces.IDataManager"""
         # set local roles before setting the value so we still have access to the old value
         roles_to_assign = self.field.roles_to_assign
-        principal_ids = self.context.acl_users.getUserIds() + self.context.acl_users.getGroupIds()
+
         # ---1 --- first find assigned roles to remove
         # it is not that easy to remove local roles because no helper method exists for removing
         # some specific local roles, only a method for removing every local roles for a list of principals...
@@ -89,10 +89,17 @@ class LocalRolesToPrincipalsDataManager(AttributeField):
                     self.context.manage_delLocalRoles((principal,))
         # ---2 --- now add new local roles
         added_principals = set(value).difference(set(old_value))
-        for added_principal in added_principals:
-            if not added_principal in principal_ids:
-                continue
-            self.context.manage_addLocalRoles(added_principal, roles_to_assign)
+
+        if added_principals:
+            # get principals, to avoid adding fake ones
+            factory = getUtility(IVocabularyFactory, 'plone.principalsource.Principals')
+            principals = factory(self.context)
+
+            for added_principal in added_principals:
+                if added_principal not in principals:
+                    continue
+                self.context.manage_addLocalRoles(added_principal, roles_to_assign)
+
         # finally set the value
         super(LocalRolesToPrincipalsDataManager, self).set(value)