[racket] scribble ++xref-in and PDF

Scribble chokes whenever I use ++xref-in setup/xref 
load-collections-xref and --pdf.  Here's an example:

$ cat try.scrbl
#lang scribble/manual
@title{Try}
@secref["numbers" #:doc '(lib "scribblings/guide/guide.scrbl")]

$ raco scribble ++xref-in setup/xref load-collections-xref --html try.scrbl
  [Output to try.html]

$ raco scribble ++xref-in setup/xref load-collections-xref --pdf try.scrbl
cadr: expects argument of type <cadrable value>; given '#((doc #"guide" 
#"numbers.html") ("Numbers") #t (part "numbers"))

  === context ===
/Users/dvanhorn/Documents/git/racket/collects/scribble/latex-render.rkt:237:4: 
render-content method in ...ble/latex-render.rkt:47:2
/Users/dvanhorn/Documents/git/racket/collects/scribble/base-render.rkt:693:4: 
render-content method in render%
...

Is this a bug or am I doing something wrong?

David