diff --git a/django/contrib/flatpages/forms.py b/django/contrib/flatpages/forms.py
index 3916c85a89..24a6e7b58e 100644
--- a/django/contrib/flatpages/forms.py
+++ b/django/contrib/flatpages/forms.py
@@ -24,7 +24,7 @@ class FlatpageForm(forms.ModelForm):
             for site in sites:
                 if same_url.filter(sites=site).exists():
                     raise forms.ValidationError(
-                        _('Flatpage with url %s already exists for site %s'
-                          % (url, site)))
+                        _('Flatpage with url %(url)s already exists for site %(site)s' %
+                          {'url': url, 'site': site}))
 
         return super(FlatpageForm, self).clean()