[plt-scheme] Name clash with mzscheme identifiers

$ cat foo.ss
(module foo mzscheme
   (define version "1.0")
   (provide version)
)
$ cat bar.ss
(require (prefix foo: "foo.ss"))

$ mzscheme -r bar.ss
foo.ss:2:2: module: identifier is already imported at: version in: 
(define-values (version) "1.0")

How can I avoid name clashes with the built-in names ?