diff --git a/wagtail/tests/testapp/models.py b/wagtail/tests/testapp/models.py
index 543c6f6261..9ee63fe86a 100644
--- a/wagtail/tests/testapp/models.py
+++ b/wagtail/tests/testapp/models.py
@@ -466,7 +466,7 @@ FormPageWithCustomSubmission.content_panels = [
 
 
 class FormFieldWithCustomSubmission(AbstractFormField):
-    page = ParentalKey(FormPageWithCustomSubmission, related_name='custom_form_fields')
+    page = ParentalKey(FormPageWithCustomSubmission, on_delete=models.CASCADE, related_name='custom_form_fields')
 
 
 class CustomFormPageSubmission(AbstractFormSubmission):