diff --git a/src/openklant/components/klanten/api/tests/test_klant.py b/src/openklant/components/klanten/api/tests/test_klant.py
index 2565c4a4..7a5a0ed7 100644
--- a/src/openklant/components/klanten/api/tests/test_klant.py
+++ b/src/openklant/components/klanten/api/tests/test_klant.py
@@ -377,7 +377,16 @@ def test_create_klant_website_url_optional(self):
 
         self.assertEqual(klant.bronorganisatie, "950428139")
         self.assertEqual(klant.website_url, "")
-        self.assertTrue(klant.klantnummer != 0)
+        self.assertEqual(klant.klantnummer, "1")
+
+        with requests_mock.Mocker() as m:
+            m.get(SUBJECT, json={})
+            response = self.client.post(list_url, data)
+
+        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
+
+        klant = Klant.objects.get(klantnummer__gt=1)
+        self.assertEqual(klant.klantnummer, "2")
 
     def test_create_klant_website_url_duplicate_klantnummer(self):
         list_url = reverse(Klant)
diff --git a/src/openklant/components/klanten/models/klanten.py b/src/openklant/components/klanten/models/klanten.py
index 8806dddd..0c16ed9d 100644
--- a/src/openklant/components/klanten/models/klanten.py
+++ b/src/openklant/components/klanten/models/klanten.py
@@ -15,8 +15,10 @@
 
 class KlantManager(models.Manager):
     def get_next_klantnummer(self):
-        id_max = Klant.objects.all().aggregate(Max("klantnummer"))["id__max"]
-        return id_max + 1 if id_max else 1
+        id_max = Klant.objects.all().aggregate(models.Max("klantnummer"))[
+            "klantnummer__max"
+        ]
+        return int(id_max) + 1 if id_max else 1
 
 
 class Klant(APIMixin, models.Model):
@@ -102,6 +104,8 @@ class Meta:
         verbose_name_plural = "klanten"
 
     def save(self, *args, **kwargs):
+        if not self.klantnummer:
+            self.klantnummer = Klant.objects.get_next_klantnummer()
         if not self.pk:
             if Klant.objects.filter(klantnummer=self.klantnummer):
                 raise Conflict("Klantnummer bestaat al")