diff --git a/synapse/server.py b/synapse/server.py
index c78eb2fe46..9055b97ac3 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -163,6 +163,8 @@ def cache_in_self(builder: T) -> T:
 
         return dep
 
+    # We cast here as we need to tell mypy that `_get` has the same signature as
+    # `builder`.
     return cast(T, _get)