diff --git a/bmspy/client.py b/bmspy/client.py
index ff6679c..1a64c92 100644
--- a/bmspy/client.py
+++ b/bmspy/client.py
@@ -45,6 +45,10 @@ def handle_registration(socket_path: str, client_name: str, debug: int = 0) -> d
     return data
 
 
+class DaemonError(RuntimeError):
+    """The daemon answered, but it refused the request."""
+
+
 def socket_comms(socket_path: str, request_data: dict[str, Any], debug: int = 0) -> dict[str, Any]:
     """Send a JSON request to the daemon socket and return the decoded JSON response."""
     response_data: dict[str, Any] = dict()
@@ -111,6 +115,15 @@ def socket_comms(socket_path: str, request_data: dict[str, Any], debug: int = 0)
     if debug > 2:
         debugger("socket client: received {!r}".format(response_data))
 
+    # For example, a daemon that is older than this client and does not know
+    # the command.
+    if isinstance(response_data, dict) and response_data.get("status") == "ERROR":
+        raise DaemonError(
+            "daemon refused {!r}: {}".format(
+                request_data.get("command"), response_data.get("message")
+            )
+        )
+
     return response_data
 
 
diff --git a/tests/test_client.py b/tests/test_client.py
index e2e7b4f..a54c44f 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -91,6 +91,28 @@ class TestReadData:
             call_args = mock_comms.call_args[0][1]
             assert call_args.get("ups") == "myups"
 
+    @pytest.mark.parametrize("call", [
+        lambda client: client.list_devices("/fake/socket", "test"),
+        lambda client: client.read_data("/fake/socket", "test"),
+    ], ids=["list_devices", "read_data"])
+    def test_error_reply_raises_the_daemon_message(self, call):
+        """An ERROR reply must not reach the code that expects device data."""
+        from bmspy import client
+        reply = json.dumps({"status": "ERROR", "message": "unknown command: 'DEVICES'"}).encode()
+        sock = MagicMock()
+        sock.recv.side_effect = [struct.pack("!I", len(reply)), reply]
+        with patch("bmspy.client.socket.socket", return_value=sock):
+            with pytest.raises(client.DaemonError, match="unknown command: 'DEVICES'"):
+                call(client)
+        sock.close.assert_called()
+
+    def test_registration_logs_an_error_reply(self, capsys):
+        from bmspy import client
+        with patch("bmspy.client.socket_comms", side_effect=client.DaemonError("daemon refused 'REGISTER': no")), \
+             patch("bmspy.client.is_registered", False):
+            client.handle_registration("/fake/socket", "test")
+        assert "daemon refused 'REGISTER': no" in capsys.readouterr().out
+
     def test_list_devices_returns_configured_names(self):
         from bmspy.client import list_devices
         with patch("bmspy.client.socket_comms", return_value={"devices": ["network", "nas"]}) as mock_comms:
diff --git a/tests/test_server.py b/tests/test_server.py
index f732ac5..7ddd762 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -507,6 +507,21 @@ class TestServerMain:
         finally:
             self._stop_server(sock_path, t)
 
+    def test_unknown_command_is_a_clear_error_in_the_client(self, tmp_path):
+        """The real client, against a server that does not know the command."""
+        from bmspy import client
+
+        sock_path = str(tmp_path / "server_refused.sock")
+        ready = threading.Event()
+        t = self._make_server_thread(sock_path, ready, threading.Event())
+        t.start()
+        ready.wait(timeout=2)
+        try:
+            with pytest.raises(client.DaemonError, match="daemon refused 'NEWER': unknown command: 'NEWER'"):
+                client.socket_comms(sock_path, {"command": "NEWER", "client": "test"})
+        finally:
+            self._stop_server(sock_path, t)
+
     def test_devices_command_lists_configured_devices(self, tmp_path):
         """DEVICES names every configured device, through the real client."""
         from bmspy import client