diff --git a/helpdesk_user_default_ticket_team/models/helpdesk_ticket.py b/helpdesk_user_default_ticket_team/models/helpdesk_ticket.py
index dd7dd0e..6610edb 100644
--- a/helpdesk_user_default_ticket_team/models/helpdesk_ticket.py
+++ b/helpdesk_user_default_ticket_team/models/helpdesk_ticket.py
@@ -20,7 +20,8 @@ class HelpdeskTicket(models.Model):
                 partner = self.env["res.partner"].browse(vals.get("partner_id"))
                 if not partner:
                     continue
-                user = self.env["res.users"].browse(partner.user_ids[0].id)
+                # A partner may have no linked user (e.g. a plain contact)
+                user = partner.user_ids[:1]
                 if not user:
                     continue