diff --git a/playbooks/roles/mailman3/files/web-settings_local.py b/playbooks/roles/mailman3/files/web-settings_local.py
index a755892a8a..05ad1eeea7 100644
--- a/playbooks/roles/mailman3/files/web-settings_local.py
+++ b/playbooks/roles/mailman3/files/web-settings_local.py
@@ -18,4 +18,8 @@ MAILMAN_WEB_SOCIAL_AUTH = []
 
 FILTER_VHOST = True
 
-DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'
+# See
+#  https://docs.djangoproject.com/en/3.2/releases/3.2/#customizing-type-of-auto-created-primary-keys
+# We set this to AutoField to avoid unwanted migrations as we've
+# already created the models with the smaller field size.
+DEFAULT_AUTO_FIELD = 'django.db.models.AutoField'