diff --git a/wagtail/tests/search/migrations/0001_initial.py b/wagtail/tests/search/migrations/0001_initial.py
index 71d3928370..018bcfdb9c 100644
--- a/wagtail/tests/search/migrations/0001_initial.py
+++ b/wagtail/tests/search/migrations/0001_initial.py
@@ -48,7 +48,7 @@ class Migration(migrations.Migration):
             fields=[
                 ('book_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='searchtests.Book')),
                 ('setting', models.CharField(max_length=255)),
-                ('protagonist', models.OneToOneField(null=True, on_delete=django.db.models.deletion.CASCADE, related_name='+', to='searchtests.Character')),
+                ('protagonist', models.OneToOneField(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+', to='searchtests.Character')),
             ],
             bases=('searchtests.book',),
         ),
diff --git a/wagtail/tests/search/models.py b/wagtail/tests/search/models.py
index eba71e8eb7..5d9baf68fb 100644
--- a/wagtail/tests/search/models.py
+++ b/wagtail/tests/search/models.py
@@ -71,7 +71,7 @@ class Book(index.Indexed, models.Model):
 
 class Character(models.Model):
     name = models.CharField(max_length=255)
-    novel = models.ForeignKey('Novel', related_name='characters')
+    novel = models.ForeignKey('Novel', related_name='characters', on_delete=models.CASCADE)
 
     search_fields = [
         index.SearchField('name'),
@@ -83,7 +83,7 @@ class Character(models.Model):
 
 class Novel(Book):
     setting = models.CharField(max_length=255)
-    protagonist = models.OneToOneField(Character, related_name='+', null=True)
+    protagonist = models.OneToOneField(Character, related_name='+', null=True, on_delete=models.SET_NULL)
 
     search_fields = Book.search_fields + [
         index.SearchField('setting', partial_match=True),