diff --git a/racketmq/hub/remote-topic.rkt b/racketmq/hub/remote-topic.rkt
index 0c8d51f..a9d4482 100644
--- a/racketmq/hub/remote-topic.rkt
+++ b/racketmq/hub/remote-topic.rkt
@@ -168,10 +168,11 @@
       (on (web-request-get (id req) (vh host-name port) ("sub" (,sub-id ())))
           (log-info "Received verification-of-intent: ~v" (web-request-header-query req))
           (define challenge (dict-ref (web-request-header-query req) 'hub.challenge ""))
-          (define lease-seconds (dict-ref (web-request-header-query req) 'hub.lease_seconds #f))
-          (if lease-seconds
+          (define lease-seconds-str (dict-ref (web-request-header-query req) 'hub.lease_seconds #f))
+          (if lease-seconds-str
               (next-subscription-refresh (+ (current-inexact-milliseconds)
-                                            (* 1000.0 (shrink-lease lease-seconds))))
+                                            (* 1000.0
+                                               (shrink-lease (string->number lease-seconds-str)))))
               (log-warning "Upstream hub for topic ~s did not supply hub.lease_seconds" full-topic))
           (web-respond/bytes! id (string->bytes/utf-8 challenge)))