[plt-scheme] namespace-variable-value

Based on my understanding of the section "8.3 Namespace Utilities" from the
docs, I was expecting the following to print a "1":

(module lang mzscheme
  (provide (all-defined)
           (all-from mzscheme))
  (define f 1))

(module test lang
  (provide (all-defined))
  (define (test1)
    (display (namespace-variable-value 'f))))

(require test)
(test1)     ;; "namespace-variable-value: f is not defined"

Why doesn't the call to namespace-variable-value in module test succeed?

Wayne