Steel Bank Common Lisp

I have a function that is generating the "could not optimize away
%SAP-ALIEN: forced to do runtime allocation of alien-value structure"
style warnings.   

    (defun fd-file-size (fd)
      (sb-posix:stat-size (sb-posix:fstat fd)))

I read the archives and it looks like it I need to provide some type
information to get rid of the warnings.  Is that right?  What are the
proper types?

Cheers,
Chris Dean