diff --git a/muxplex/settings.py b/muxplex/settings.py
index ba4f649..34a1f3d 100644
--- a/muxplex/settings.py
+++ b/muxplex/settings.py
@@ -26,6 +26,7 @@ DEFAULT_SETTINGS: dict = {
     "device_name": "",
     "delete_session_template": "tmux kill-session -t {name}",
     "multi_device_enabled": False,
+    "federation_key": "",
 }
 
 
diff --git a/muxplex/tests/test_settings.py b/muxplex/tests/test_settings.py
index eb0f936..1239b28 100644
--- a/muxplex/tests/test_settings.py
+++ b/muxplex/tests/test_settings.py
@@ -377,6 +377,12 @@ def test_serve_keys_patchable():
     assert loaded["session_ttl"] == 3600
 
 
+def test_defaults_include_federation_key():
+    """DEFAULT_SETTINGS must have 'federation_key' key initialised to empty string."""
+    assert "federation_key" in DEFAULT_SETTINGS
+    assert DEFAULT_SETTINGS["federation_key"] == ""
+
+
 def test_old_settings_file_without_serve_keys_loads_correctly(redirect_settings_path):
     """Old settings.json without serve keys loads correctly with defaults filled in."""
     # Write an old-style settings file that has no serve keys