diff --git a/tests/queries/models.py b/tests/queries/models.py
index 383f633be9..c3322c224e 100644
--- a/tests/queries/models.py
+++ b/tests/queries/models.py
@@ -636,7 +636,7 @@ class StaffUser(BaseUser):
     staff = models.OneToOneField(Staff, models.CASCADE, related_name='user')
 
     def __str__(self):
-        return self.staff
+        return str(self.staff)
 
 
 class Ticket21203Parent(models.Model):