diff --git a/resource_multi_week_work_time_from_contracts/models/resource_mixin.py b/resource_multi_week_work_time_from_contracts/models/resource_mixin.py
index b9c22acc..3ef91588 100644
--- a/resource_multi_week_work_time_from_contracts/models/resource_mixin.py
+++ b/resource_multi_week_work_time_from_contracts/models/resource_mixin.py
@@ -10,4 +10,6 @@ class ResourceMixing(models.AbstractModel):
 
     def get_calendar_for_date(self, date):
         calendar = super().get_calendar_for_date(date)
-        return calendar._get_multi_week_calendar(day=date)
+        if calendar is not None:
+            calendar = calendar._get_multi_week_calendar(day=date)
+        return calendar