diff --git a/corres2math/matrix.py b/corres2math/matrix.py
index cb1e0e6..1f340d8 100644
--- a/corres2math/matrix.py
+++ b/corres2math/matrix.py
@@ -18,11 +18,14 @@ class Matrix:
     _device_id: str = None
 
     @classmethod
-    async def _get_client(cls) -> AsyncClient:
+    async def _get_client(cls) -> Union[AsyncClient, "FakeMatrixClient"]:
         """
         Retrieve the bot account.
         If not logged, log in and store access token.
         """
+        if not os.getenv("SYNAPSE_PASSWORD"):
+            return FakeMatrixClient()
+
         client = AsyncClient("https://correspondances-maths.fr", "@corres2mathbot:correspondances-maths.fr")
         client.user_id = "@corres2mathbot:correspondances-maths.fr"
 
@@ -212,9 +215,9 @@ class Matrix:
         """
         client = await cls._get_client()
         resp: RoomResolveAliasResponse = await client.room_resolve_alias(room_alias)
-        if isinstance(resp, RoomResolveAliasError):
-            return None
-        return resp.room_id
+        if isinstance(resp, RoomResolveAliasResponse):
+            return resp.room_id
+        return None
 
     @classmethod
     @async_to_sync
@@ -328,9 +331,20 @@ class Matrix:
             avatar_uri (str): The internal avatar URI to apply.
         """
         client = await cls._get_client()
-        client
         if room_id.startswith("#"):
             room_id = await cls.resolve_room_alias(room_id)
         return await client.room_put_state(room_id, "m.room.avatar", content={
             "url": avatar_uri
         }, state_key="")
+
+
+class FakeMatrixClient:
+    """
+    Simulate a Matrix client to run tests, if no Matrix homeserver is connected.
+    """
+
+    def __getattribute__(self, item):
+        async def func(*_, **_2):
+            return None
+        return func
+