diff --git a/app/auth/models.py b/app/auth/models.py
index 02b0c5e72..8b3597565 100644
--- a/app/auth/models.py
+++ b/app/auth/models.py
@@ -250,7 +250,7 @@ class User(UserMixin, db.Model):
         """string repr. of user's roles (with depts)
         e.g. "Ens_RT, Ens_Info, Secr_CJ"
         """
-        return ",".join("{r.role.name}_{r.dept}".format(r=r) for r in self.user_roles)
+        return ",".join(f"{r.role.name}_{r.dept or ''}" for r in self.user_roles)
 
     def is_administrator(self):
         "True if i'm an active SuperAdmin"
@@ -400,15 +400,17 @@ class UserRole(db.Model):
         return "<UserRole u={} r={} dept={}>".format(self.user, self.role, self.dept)
 
     @staticmethod
-    def role_dept_from_string(role_dept):
+    def role_dept_from_string(role_dept: str):
         """Return tuple (role, dept) from the string
         role_dept, of the forme "Role_Dept".
-        role is a Role instance, dept is a string.
+        role is a Role instance, dept is a string, or None.
         """
         fields = role_dept.split("_", 1)  # maxsplit=1, le dept peut contenir un "_"
         if len(fields) != 2:
             raise ScoValueError("Invalid role_dept")
         role_name, dept = fields
+        if dept == "":
+            dept = None
         role = Role.query.filter_by(name=role_name).first()
         if role is None:
             raise ScoValueError("role %s does not exists" % role_name)