diff --git a/app/auth/models.py b/app/auth/models.py
index d3548cb1..544afc31 100644
--- a/app/auth/models.py
+++ b/app/auth/models.py
@@ -173,7 +173,7 @@ class User(UserMixin, db.Model):
             "id": self.id,
             "active": self.active,
             "status_txt": "actif" if self.active else "fermé",
-            "last_seen": self.last_seen.isoformat() + "Z",
+            "last_seen": self.last_seen.isoformat() + "Z" if self.last_seen else "",
             "nom": (self.nom or ""),  # sco8
             "prenom": (self.prenom or ""),  # sco8
             "roles_string": self.get_roles_string(),  # eg "Ens_RT, Ens_Info"