diff --git a/chrono/agendas/models.py b/chrono/agendas/models.py
index b974c639..50409e8d 100644
--- a/chrono/agendas/models.py
+++ b/chrono/agendas/models.py
@@ -484,10 +484,13 @@ class Desk(models.Model):
             response = requests.get(url, proxies=settings.REQUESTS_PROXIES)
             response.raise_for_status()
         except requests.HTTPError as e:
-            raise ICSError(_('Failed to retrieve remote calendar (%s, HTTP error %s).') % (
-                url, e.response.status_code))
+            raise ICSError(
+                _('Failed to retrieve remote calendar (%(url)s, HTTP error %(status_code)s).') %
+                {'url': url, 'status_code': e.response.status_code})
         except requests.RequestException as e:
-            raise ICSError(_('Failed to retrieve remote calendar (%s, %s).') % (url, e))
+            raise ICSError(
+                _('Failed to retrieve remote calendar (%(url)s, %(exception)s).') %
+                {'url': url, 'exception': e})
 
         return self.create_timeperiod_exceptions_from_ics(response.text, keep_synced_by_uid=True)