[plt-scheme] Very basic problem requiring (planet untyped/unlib/string)

The sequence

  #lang scheme
  (require (planet untyped/unlib/string))

yields the following diagnostics:

  Welcome to DrScheme, version 4.1 [3m].
  Language: Module; memory limit: 128 megabytes.
  compile: unbound variable in module
  setup-plt: error: during making for <planet>/untyped/unlib.plt/3/8 (unlib)
  setup-plt:   compile: unbound variable in module

Flushing the planet cache doesn't seem to help.  Starting from an empty 
cache (assuming that "planet show" tells the truth), the above require 
pulls in

Normally-installed packages:
  dherman       test.plt        1 3
  ryanc require.plt     1 3
  ryanc scripting.plt   1 1
  schematics    macro.plt       1 2
  schematics    namespace.plt   1 0
  schematics    schemeunit.plt  2 10
  schematics    schemeunit.plt  3 3
  untyped       unlib.plt       3 8

I'm new to planet, and not sure how to proceed from here.  The unlib.plt 
3.8 string.ss looks fine to me.  What could I do to chase this problem down?

Thanks,
Ben