diff --git a/queue_job/controllers/main.py b/queue_job/controllers/main.py
index a1729b1fe..d43498e6b 100644
--- a/queue_job/controllers/main.py
+++ b/queue_job/controllers/main.py
@@ -216,6 +216,14 @@ def _get_failure_values(cls, job, traceback_txt, orig_exception):
     def runjob(self, db, job_uuid, **kw):
         http.request.session.db = db
         env = http.request.env(user=SUPERUSER_ID)
+        # `IrHttp._auth_method_none()` (this route's auth method) forces
+        # `transaction.default_env` to an environment with `uid=None`,
+        # bypassing the truthy-uid guard in `Environment.__new__`. Any
+        # `env.user` access during a savepoint-triggered flush inside the
+        # job (e.g. a recompute/constrain calling `has_group`) then raises
+        # "Expected singleton: res.users()" because `browse(None)` is
+        # empty. Repair it to the real job-running user.
+        env.transaction.default_env = env
         job = self._acquire_job(env, job_uuid)
         if not job:
             return ""
diff --git a/queue_job/tests/test_run_rob_controller.py b/queue_job/tests/test_run_rob_controller.py
index 1a15f4363..9ec96ccc1 100644
--- a/queue_job/tests/test_run_rob_controller.py
+++ b/queue_job/tests/test_run_rob_controller.py
@@ -1,9 +1,10 @@
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
 
-from odoo.tests.common import TransactionCase
+from odoo.tests.common import HttpCase, TransactionCase
 
 from ..controllers.main import RunJobController
 from ..job import Job
+from ..models.queue_job import QueueJob
 
 
 class TestRunJobController(TransactionCase):
@@ -21,3 +22,34 @@ def test_runjob_success(self):
         RunJobController._runjob(self.env, job)
         self.assertEqual(job.state, "done")
         self.assertEqual(job.db_record().state, "done")
+
+
+class TestRunJobControllerHttp(HttpCase):
+    def test_runjob_http_repairs_default_env(self):
+        """A job run through the real /queue_job/runjob (auth=none) route
+        must leave `transaction.default_env` on a valid, real user.
+
+        `IrHttp._auth_method_none()` forces `transaction.default_env` to an
+        environment with `uid=None` before the controller runs. Any
+        `env.user` access during a savepoint-triggered flush inside the job
+        (e.g. a recompute/constrain calling `has_group`) then raises
+        "Expected singleton: res.users()" because `browse(None)` is empty.
+        """
+
+        def _test_job(self):
+            with self.env.cr.savepoint():
+                pass
+            # Would raise "Expected singleton: res.users()" without the fix.
+            self.env.transaction.default_env.user.ensure_one()
+
+        self.patch(QueueJob, "_test_job", _test_job)
+        job_ = self.env["queue.job"].with_delay()._test_job()
+        job_.set_enqueued()
+        job_.store()
+        db_job = job_.db_record()
+        response = self.url_open(
+            f"/queue_job/runjob?db={self.env.cr.dbname}&job_uuid={db_job.uuid}"
+        )
+        self.assertEqual(response.status_code, 200)
+        db_job.invalidate_recordset()
+        self.assertEqual(db_job.state, "done", db_job.exc_info)