diff --git a/racketmq/hub.rkt b/racketmq/hub.rkt
index 3d06127..ee59e83 100644
--- a/racketmq/hub.rkt
+++ b/racketmq/hub.rkt
@@ -24,10 +24,19 @@
                (spawn-configuration filename)])
 
 (actor #:name 'main
+
        (during (config (list 'canonical-host $h $p))
          (assert (canonical-local-host h p))
          (assert (local-host h p)))
+
+       (define/query-set canonical-local-hosts ($ c (canonical-local-host _ _)) c)
+       (stop-when (rising-edge (> (set-count (canonical-local-hosts)) 1))
+                  (log-error "Too many canonical-host records in configuration."))
+       ;; TODO: Make the too-many-canonical-host-records situation recoverable.
+       ;; TODO: And/or, make the whole application quit when it gets into a bad state.
+
        (during (config (list 'accepted-host $h $p))
          (assert (local-host h p)))
+
        (during (local-host $host-name $port)
          (assert (vh host-name port))))