diff --git a/src/evohomeasync/auth.py b/src/evohomeasync/auth.py
index 710ea37..c95145f 100644
--- a/src/evohomeasync/auth.py
+++ b/src/evohomeasync/auth.py
@@ -213,6 +213,7 @@ def __init__(
     async def _headers(self, headers: dict[str, str] | None = None) -> dict[str, str]:
         """Ensure the authorization header has a valid session id."""
 
-        return (headers or HEADERS_BASE) | {
+        headers = HEADERS_BASE | (headers or {})
+        return headers | {
             "sessionId": await self._get_session_id(),
         }
diff --git a/src/evohomeasync2/auth.py b/src/evohomeasync2/auth.py
index 77c1fc5..d9d34c8 100644
--- a/src/evohomeasync2/auth.py
+++ b/src/evohomeasync2/auth.py
@@ -269,6 +269,7 @@ def __init__(
     async def _headers(self, headers: dict[str, str] | None = None) -> dict[str, str]:
         """Ensure the authorization header has a valid access token."""
 
-        return (headers or HEADERS_BASE) | {
+        headers = HEADERS_BASE | (headers or {})
+        return headers | {
             "Authorization": "bearer " + await self._get_access_token(),
         }
diff --git a/tests/tests_rf/test_v2_urlz.py b/tests/tests_rf/test_v2_urlz.py
index 3400c5f..4eec5cb 100644
--- a/tests/tests_rf/test_v2_urlz.py
+++ b/tests/tests_rf/test_v2_urlz.py
@@ -131,6 +131,8 @@ async def test_tcs_urls(
         json={"SystemMode": "Auto", "Permanent": True},
     )  # type: ignore[assignment]
 
+    # factory_tcs_status()(task)  # {'id': '1668279943'}
+
 
 @skipif_auth_failed
 @pytest.mark.skipif(not _DBG_USE_REAL_AIOHTTP, reason="requires vendor's webserver")