diff --git a/netbox_dns/models/zone.py b/netbox_dns/models/zone.py
index 70211419..1d6cb091 100644
--- a/netbox_dns/models/zone.py
+++ b/netbox_dns/models/zone.py
@@ -642,7 +642,7 @@ def clean(self, *args, **kwargs):
             old_zone = Zone.objects.get(pk=self.pk)
             if not self.soa_serial_auto:
                 self.check_soa_serial_increment(old_zone.soa_serial, self.soa_serial)
-            else:
+            elif not old_zone.soa_serial_auto:
                 try:
                     self.check_soa_serial_increment(
                         old_zone.soa_serial, self.get_auto_serial()