Fixed #18234: Avoided exception on flatpage add

django/contrib/flatpages/forms.py

@@ -31,7 +31,7 @@ class FlatpageForm(forms.ModelForm):
         if self.instance.pk:
             same_url = same_url.exclude(pk=self.instance.pk)
 
-        if same_url.filter(sites__in=sites).exists():
+        if sites and same_url.filter(sites__in=sites).exists():
             for site in sites:
                 if same_url.filter(sites=site).exists():
                     raise forms.ValidationError(

django/contrib/flatpages/tests/forms.py

@@ -78,3 +78,16 @@ class FlatpageAdminFormTests(TestCase):
         updated = f.save()
 
         self.assertEqual(updated.title, "A test page")
+
+    def test_flatpage_nosites(self):
+        data = dict(url='/myflatpage1/', **self.form_data)
+        data.update({'sites': ''})
+
+        f = FlatpageForm(data=data)
+
+        self.assertFalse(f.is_valid())
+
+        self.assertEqual(
+            f.errors,
+            {'sites': [u'This field is required.']})
+