diff --git a/ops/model.py b/ops/model.py
index be11dffb8..f04b95b4e 100644
--- a/ops/model.py
+++ b/ops/model.py
@@ -1791,6 +1791,8 @@ class UnknownStatus(StatusBase):
     A unit-agent has finished calling install, config-changed and start, but the
     charm has not called status-set yet.
 
+    This status is read-only; trying to set unit or application status to
+    ``UnknownStatus`` will raise :class:`ModelError`.
     """
     name = 'unknown'
 
@@ -1808,6 +1810,9 @@ class ErrorStatus(StatusBase):
 
     The unit-agent has encountered an error (the application or unit requires
     human intervention in order to operate correctly).
+
+    This status is read-only; trying to set unit or application status to
+    ``ErrorStatus`` will raise :class:`ModelError`.
     """
     name = 'error'
 
diff --git a/ops/testing.py b/ops/testing.py
index 19d44fab6..e399747f3 100644
--- a/ops/testing.py
+++ b/ops/testing.py
@@ -448,7 +448,7 @@ def begin_with_initial_hooks(self) -> None:
         # the unit status from "Maintenance" to "Unknown". See gh#726
         post_setup_sts = self._backend.status_get()
         if post_setup_sts.get("status") == "maintenance" and not post_setup_sts.get("message"):
-            self._backend.status_set("unknown", "", is_app=False)
+            self._backend._unit_status = {'status': 'unknown', 'message': ''}
         all_ids = list(self._backend._relation_names.items())
         random.shuffle(all_ids)
         for rel_id, rel_name in all_ids:
@@ -2256,6 +2256,9 @@ def status_get(self, *, is_app: bool = False):
             return self._unit_status
 
     def status_set(self, status: '_StatusName', message: str = '', *, is_app: bool = False):
+        if status in [model.ErrorStatus.name, model.UnknownStatus.name]:
+            raise model.ModelError(f'ERROR invalid status "{status}", expected one of'
+                                   ' [maintenance blocked waiting active]')
         if is_app:
             self._app_status = {'status': status, 'message': message}
         else:
diff --git a/test/test_testing.py b/test/test_testing.py
index 1921daf04..f515f618b 100644
--- a/test/test_testing.py
+++ b/test/test_testing.py
@@ -2851,6 +2851,23 @@ def _on_collect_unit_status(self, event: ops.CollectStatusEvent):
         self.assertEqual(harness.model.app.status, ops.ActiveStatus('active app'))
         self.assertEqual(harness.model.unit.status, ops.ActiveStatus('active unit'))
 
+    def test_invalid_status_set(self):
+        harness = ops.testing.Harness(ops.CharmBase)
+        harness.set_leader(True)
+        harness.begin()
+
+        with self.assertRaises(ops.model.ModelError):
+            harness.model.app.status = ops.UnknownStatus()
+        with self.assertRaises(ops.model.ModelError):
+            harness.model.app.status = ops.ErrorStatus()
+        harness.model.app.status = ops.ActiveStatus()
+
+        with self.assertRaises(ops.model.ModelError):
+            harness.model.unit.status = ops.UnknownStatus()
+        with self.assertRaises(ops.model.ModelError):
+            harness.model.unit.status = ops.ErrorStatus()
+        harness.model.unit.status = ops.ActiveStatus()
+
 
 class TestNetwork(unittest.TestCase):
     def setUp(self):