diff --git a/app/models/communication/website/agenda/event/with_kinds.rb b/app/models/communication/website/agenda/event/with_kinds.rb
index f4aa1408fb..8a01cadf81 100644
--- a/app/models/communication/website/agenda/event/with_kinds.rb
+++ b/app/models/communication/website/agenda/event/with_kinds.rb
@@ -120,6 +120,8 @@ def no_child_after?
   end
 
   def in_parent_dates?
+    return if from_day.nil? || to_day.nil?
+
     if from_day < parent.from_day || to_day > parent.to_day
       errors.add(:parent_id, :outside_parent_dates)
     end
diff --git a/test/models/communication/website/agenda/event_test.rb b/test/models/communication/website/agenda/event_test.rb
index 84b8b5e6d5..dab64f41a8 100644
--- a/test/models/communication/website/agenda/event_test.rb
+++ b/test/models/communication/website/agenda/event_test.rb
@@ -81,6 +81,21 @@ class Communication::Website::Agenda::EventTest < ActiveSupport::TestCase
     assert_nothing_raised { event_l10n.cal }
   end
 
+  test "child event with a missing start date is invalid, not a crash" do
+    parent = new_event(
+      from_day: Date.tomorrow,
+      to_day: Date.tomorrow + 2.days
+    )
+    parent.save!
+
+    child = new_event(
+      from_day: nil,
+      parent: parent
+    )
+    assert_nothing_raised { child.valid? }
+    assert_not(child.valid?)
+  end
+
   protected
 
   def new_event(**options)