diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 67a89a1d72..6bc3479755 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -147,6 +147,10 @@ class IdentityHandler(BaseHandler):
         if not self.trusted_id_servers:
             logger.warn("Can't unbind threepid: no trusted ID servers set in config")
             defer.returnValue(False)
+
+        # We don't track what ID server we added 3pids on (perhaps we ought to) but we assume
+        # that any of the servers in the trusted list are in the same ID server federation,
+        # so we can pick any one of them to send the deletion request to.
         id_server = next(iter(self.trusted_id_servers))
 
         url = "https://%s/_matrix/identity/api/v1/3pid/unbind" % (id_server,)