diff --git a/docs/history.rst b/docs/history.rst
index e38664e..989140e 100644
--- a/docs/history.rst
+++ b/docs/history.rst
@@ -1,5 +1,8 @@
 History
 =======
+4.0.6
+  * bugfix: default GMCP modules requested are now in lowercase instead of titlecase
+
 4.0.5
   * enhancement: ``telnetlib3-client`` client shell now drains stdout.
 
diff --git a/pyproject.toml b/pyproject.toml
index 9d4f131..d1e4ccb 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "telnetlib3"
-version = "4.0.5"  # Keep in sync with telnetlib3/accessories.py::get_version !
+version = "4.0.6"  # Keep in sync with telnetlib3/accessories.py::get_version !
 description = " Python Telnet server and client CLI and Protocol library"
 readme = "README.rst"
 license = "ISC"
diff --git a/telnetlib3/accessories.py b/telnetlib3/accessories.py
index 27de148..078fb1b 100644
--- a/telnetlib3/accessories.py
+++ b/telnetlib3/accessories.py
@@ -42,7 +42,7 @@
 
 def get_version() -> str:
     """Return the current version of telnetlib3."""
-    return "4.0.5"  # keep in sync with pyproject.toml !
+    return "4.0.6"  # keep in sync with pyproject.toml !
 
 
 def encoding_from_lang(lang: str) -> Optional[str]:
diff --git a/telnetlib3/client.py b/telnetlib3/client.py
index 52b504a..9eb0c83 100755
--- a/telnetlib3/client.py
+++ b/telnetlib3/client.py
@@ -26,14 +26,14 @@
 #: Sub-modules are listed explicitly because not all servers treat
 #: top-level subscriptions as wildcards.
 _DEFAULT_GMCP_MODULES = [
-    "Char 1",
-    "Char.Vitals 1",
-    "Char.Items 1",
-    "Room 1",
-    "Room.Info 1",
-    "Comm 1",
-    "Comm.Channel 1",
-    "Group 1",
+    "char 1",
+    "char.vitals 1",
+    "char.items 1",
+    "room 1",
+    "room.info 1",
+    "comm 1",
+    "comm.channel 1",
+    "group 1",
 ]
 
 
@@ -197,8 +197,9 @@ def _send_gmcp_hello(self) -> None:
         from telnetlib3.accessories import get_version
 
         self.writer.send_gmcp("Core.Hello", {"client": "telnetlib3", "version": get_version()})
-        self.writer.send_gmcp("Core.Supports.Set", self._gmcp_modules)
-        self.log.info("GMCP handshake: Core.Hello + Core.Supports.Set %s", self._gmcp_modules)
+        wire_modules = [m.lower() for m in self._gmcp_modules]
+        self.writer.send_gmcp("Core.Supports.Set", wire_modules)
+        self.log.info("GMCP handshake: Core.Hello + Core.Supports.Set %s", wire_modules)
 
     def on_gmcp(self, package: str, data: Any) -> None:
         """Store incoming GMCP data on ``writer.ctx``, merging dict updates."""
diff --git a/telnetlib3/tests/test_client_unit.py b/telnetlib3/tests/test_client_unit.py
index 70cce62..3082260 100644
--- a/telnetlib3/tests/test_client_unit.py
+++ b/telnetlib3/tests/test_client_unit.py
@@ -2,6 +2,7 @@
 import sys
 import types
 import asyncio
+from unittest import mock
 
 # 3rd party
 import pytest
@@ -516,6 +517,52 @@ async def test_on_gmcp_merges_dicts_on_writer_ctx():
     assert client.writer.ctx.gmcp_data["Char.Vitals"] == {"hp": 63, "maxhp": 100}
 
 
+def test_default_gmcp_modules_are_lowercase():
+    for spec in cl._DEFAULT_GMCP_MODULES:
+        module_part = spec.rsplit(" ", 1)[0]
+        assert module_part == module_part.lower()
+
+
+@pytest.mark.asyncio
+async def test_send_gmcp_hello_lowercases_default_modules():
+    client = _make_client()
+    calls: list[tuple[str, object]] = []
+    mock_writer = mock.Mock()
+    mock_writer.send_gmcp.side_effect = lambda pkg, data: calls.append((pkg, data))
+    client.writer = mock_writer
+    client._send_gmcp_hello()
+    assert client._gmcp_hello_sent is True
+    assert len(calls) == 2
+    pkg_name, supports_set = calls[1]
+    assert pkg_name == "Core.Supports.Set"
+    for spec in supports_set:
+        assert spec == spec.lower()
+
+
+@pytest.mark.asyncio
+async def test_send_gmcp_hello_lowercases_consumer_modules():
+    client = _make_client(gmcp_modules=["Room.Info 1", "Char 1"])
+    calls: list[tuple[str, object]] = []
+    mock_writer = mock.Mock()
+    mock_writer.send_gmcp.side_effect = lambda pkg, data: calls.append((pkg, data))
+    client.writer = mock_writer
+    client._send_gmcp_hello()
+    _, supports_set = calls[1]
+    assert "room.info 1" in supports_set
+    assert "char 1" in supports_set
+
+
+@pytest.mark.asyncio
+async def test_send_gmcp_hello_idempotent():
+    client = _make_client()
+    mock_writer = mock.Mock()
+    client.writer = mock_writer
+    client._send_gmcp_hello()
+    call_count = mock_writer.send_gmcp.call_count
+    client._send_gmcp_hello()
+    assert mock_writer.send_gmcp.call_count == call_count
+
+
 def test_fingerprint_main_oserror(monkeypatch):
     async def _bad_fp():
         raise OSError("connection refused")