[plt-scheme] "require" introduces additional bindings in language modules

Why doesn't the following cause an error?

(module mylang mzscheme
  (provide #%module-begin #%top require))
(module test-mylang mylang
  (require (only mzscheme add1))
  sub1)   <------ sub1 should be undefined here!

If I remove the require, or I just use regular modules instead of a
language module, then I get an error message as expected.  Is there
some subtle interaction between #%module-begin and require that I'm
interfering with?

-- 
Kimberley Burchett
http://www.kimbly.com/