diff --git a/shifts.py b/shifts.py
index cd3f60c..a58db1a 100644
--- a/shifts.py
+++ b/shifts.py
@@ -685,7 +685,7 @@ class RotaBuilder(object):
                 self.model.require_presence_at_site_overnight_rule = Constraint(
                     [week for week in self.weeks],
                     [shift.name for shift in self.get_shifts_with_constraint("night")],
-                    site,
+                    [site],
                     rule=presenceAtSiteOvernight,
                 )