[racket] Help debugging a ffi crash

I'm trying to debug an FFI crash that I'm seeing, and because it is
dealing with C code the error just presents as a segfault. I believe I
have tracked down what is causing the problem, but don't understand
how it could be doing so.

I have two racket functions which take a "cursor" (the foreign
libraries object) and return a string representation of it, which I'm
trying to use for debugging.

(define raw-clang-get-cstring
  (get-ffi-obj "clang_getCString" lib-clang
    (_fun _pointer -> _string)))

(define raw-cursor-spelling
 (get-ffi-obj "clang_getCursorSpelling" lib-clang
   (_fun _CXCursor -> _pointer)))

(define (cursor-spelling c)
  (raw-clang-get-cstring (raw-cursor-spelling c)))

(define cursor-spelling2
 (get-ffi-obj "clang_getCursorSpelling" lib-clang
   (_fun _CXCursor -> (make-ctype _pointer values (λ (v)
(raw-clang-get-cstring v))))))

If I use cursor-spelling, I have not been able to trigger a crash. But
if I use cursor-spelling2 I can reliably trigger a crash.

Is there anything obvious on how these functions are different?
Because they look to me like they should be doing the same thing. If
it would be helpful I can try to get my code in a portable enough
shape so that it will work/crash on another machine.