[plt-scheme] include and sub-directories

When I try:

     #lang scheme/base
     (require scheme/include)
     (include "foo/bar.scm")

I get the error:

     include: bad relative pathname string in: "foo/bar.scm"

I tried looking at the source but got lost once I saw it called 
`resolve-path-spec' with three arguments, which the docs say takes 
exactly four arguments. Uncle!

Thanks,
Dave