Unable to install sexp gem

I am getting error while installing sexp-0.1.gem
(RuntimeError)
Error instaling sexp-0.1.gem:
sexp requires multi = 0.1

My ruby version is
“ruby 1.8.5 (2006-12-25 patchlevel 12) [i386-mswin32]”

What could be the reason?
Please help
thanks