[plt-scheme] Odd SRFI-19 problem on OS X

(require (lib "time.ss" "srfi" "19"))
(map (lambda (x)
       (date->string
        (make-srfi:date 0 0 0 0 0 x 0 0 )
        "~m/~B"))
     `(8 9 10 11))
; => ("08/August" "09/#f" "10/#f" "11/November")

This is the latest svn on the latest OS X. 

Any ideas?

-- 
Jay McCarthy <jay at cs.brown.edu>
http://jay.makeoutcity.com/