diff --git a/src/program/migrations/0086_sessions_slots_availability.py b/src/program/migrations/0086_sessions_slots_availability.py
index bb612a24..5621789d 100644
--- a/src/program/migrations/0086_sessions_slots_availability.py
+++ b/src/program/migrations/0086_sessions_slots_availability.py
@@ -189,6 +189,7 @@ class Migration(migrations.Migration):
             model_name="event",
             name="duration_minutes",
             field=models.PositiveIntegerField(
+                null=True,
                 blank=True,
                 default=None,
                 help_text="The duration of this event in minutes. Leave blank to use the default from the eventtype.",