diff --git a/model.py b/model.py
index a716b7e..07fee8a 100644
--- a/model.py
+++ b/model.py
@@ -48,11 +48,13 @@ class Model(object):
         sessionname = generate_random_id()
         newsession = {"id": sessionname, "owner": clientid, "clients": []}
         self.model["sessions"][sessionname] = newsession
-        return sessionname
 
     @api_method()
     async def join_session(self, clientid, sessionid):
-        self.model["sessions"][sessionid]
+        if sessionid in self.model["sessions"]:
+            self.model["sessions"][sessionid].append(clientid)
+        else:
+            raise Exception(f"Session {sessionid} does not exist")
 
     async def send_state(self, clientid):
         # TODO: compute state, send to client