diff --git a/glebby-server/glebby/__init__.py b/glebby-server/glebby/__init__.py
index 7f72fb1..cc5b9e0 100644
--- a/glebby-server/glebby/__init__.py
+++ b/glebby-server/glebby/__init__.py
@@ -1,6 +1,7 @@
 import asyncio
 
 from quart import Quart, Response, websocket
+from typing import NoReturn
 
 from glebby.connection_manager import ConnectionManager
 from glebby.model import Model
@@ -20,5 +21,5 @@ async def root() -> Response:
 # is set up in the connection manager. It will handle sending and receiving
 # and keeping the model up to date.
 @app.websocket('/glebby')
-async def ws() -> None:
+async def ws() -> NoReturn:
     await connection_manager.setup_connection()
diff --git a/glebby-server/glebby/connection_manager.py b/glebby-server/glebby/connection_manager.py
index 821dc1a..a542215 100644
--- a/glebby-server/glebby/connection_manager.py
+++ b/glebby-server/glebby/connection_manager.py
@@ -3,17 +3,17 @@ import itertools
 
 from collections import OrderedDict
 from quart import websocket
-from typing import AsyncGenerator, Iterator, Optional
+from typing import Any, AsyncGenerator, Awaitable, Callable, Iterator, NoReturn, Optional
 
 # Represents a single websocket connection
 class Connection:
     outgoing_queue: asyncio.Queue[str]
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.outgoing_queue = asyncio.Queue()
     
     # Sending a message simply puts it in the outgoing queue...
-    async def send(self, data: str):
+    async def send(self, data: str) -> None:
         await self.outgoing_queue.put(data)
     
     # ... and this infinite loop takes care of actually sending the outgoing
@@ -28,7 +28,11 @@ class ConnectionManager:
     id_generator: Iterator[int]
     connections: OrderedDict[int, Connection]
 
-    def __init__(self):
+    on_open: Optional[Callable[[int], Awaitable[Any]]]
+    on_message: Optional[Callable[[int, str], Awaitable[Any]]]
+    on_close: Optional[Callable[[int], Awaitable[Any]]]
+
+    def __init__(self) -> None:
         self.id_generator = itertools.count(start=0, step=1)
         self.connections = OrderedDict()
 
@@ -38,25 +42,30 @@ class ConnectionManager:
 
     # Wires up asyncio tasks etc. such that on_open, on_message and on_close
     # are called at the right time with the right arguments.
-    async def setup_connection(self):
+    async def setup_connection(self) -> NoReturn:
         connection_id = next(self.id_generator)
         connection = Connection()
         self.connections[connection_id] = connection
 
-        await self.on_open(connection_id)
+        if self.on_open:
+            await self.on_open(connection_id)
 
         try:
             send_task = asyncio.create_task(connection._send_loop())
             while True:
                 data = await websocket.receive()
-                await self.on_message(connection_id, data)
+                if self.on_message:
+                    await self.on_message(connection_id, data)
 
         except asyncio.CancelledError:
             del self.connections[connection_id]
             send_task.cancel()
             await send_task
 
-            await self.on_close(connection_id)
+            if self.on_close:
+                await self.on_close(connection_id)
+            
+            raise
     
     # Interface for the model to send messages. The model shouldn't
     # use the Connection class at all.