diff --git a/alkatorapi/views.py b/alkatorapi/views.py
index b44bf50..93ec0ec 100644
--- a/alkatorapi/views.py
+++ b/alkatorapi/views.py
@@ -52,7 +52,7 @@ def register(request):
 
     try:
         latest_user = User.objects.latest("invoice_id")
-        if latest_user.invoice_id < invoice_id:
+        if latest_user.invoice_id is None or latest_user.invoice_id < invoice_id:
             invoice_id = invoice_id + 1
         else:
             invoice_id = latest_user.invoice_id + 1