[plt-scheme] Bug or misuse?

Is this a bug or am I misusing resolve-module-path-index ?

(Note: "stx" is the what I passed as the second argument (rel-to-path-v).

Error message:

(file "/Applications/PLT Scheme Full
v4.2.0.6/collects/syntax/modresolve.ss") broke the contract
  (->
   (or/c symbol? module-path-index?)
   (or/c (-> path-string?) path-string? #f)
   (or/c path? symbol?))
 on resolve-module-path-index; expected <(or/c path? symbol?)>, given: "stx"

--
Jens Axel Søgaard