diff --git a/src/collective/z3cform/rolefield/field.py b/src/collective/z3cform/rolefield/field.py
index 7f02f9b..70856aa 100644
--- a/src/collective/z3cform/rolefield/field.py
+++ b/src/collective/z3cform/rolefield/field.py
@@ -61,6 +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...
@@ -89,6 +90,8 @@ class LocalRolesToPrincipalsDataManager(AttributeField):
         # ---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)
         # finally set the value
         super(LocalRolesToPrincipalsDataManager, self).set(value)
diff --git a/src/collective/z3cform/rolefield/tests.py b/src/collective/z3cform/rolefield/tests.py
index 529cdd2..7c40e8a 100644
--- a/src/collective/z3cform/rolefield/tests.py
+++ b/src/collective/z3cform/rolefield/tests.py
@@ -81,3 +81,6 @@ class TestRoleField(unittest.TestCase, BaseTest):
         datamanager.set(())
         # not managed local_roles are kepts
         self.assertEquals(tuple(testingobj.__ac_local_roles__['Reviewers']), ('Reader',))
+        # add a not existing principal value, test that it is not set
+        datamanager.set(('toto',))
+        self.failIf('toto' in testingobj.__ac_local_roles__.keys())