[plt-scheme] uncertified context error

Can someone explain this error, and how to avoid it?  Thanks -- David

$ cat host-url.ss
(module host-url mzscheme
   (provide host-url)
   (require (lib "url.ss" "net"))
   (define host-url (string->url "http://riverbot.com/")))

$ mzc host-url.ss
MzScheme compiler (mzc) version 299.24, Copyright (c) 2004 PLT Scheme, Inc.
"host-url.ss":
  [?,?]: Error: Error during elaboration: compile: access from an uncertified 
context to unexported variable from module: 
|,/local/riverbot/local/plt/collects/net/url| at: 
provide/contract-contract-id-string->url in: 
provide/contract-contract-id-string->url.77
Errors encountered.  Compilation aborted.