Autoload with rubygems?

I’m sure this has been asked before but I couldn’t find an actual
answer.

This works:

require ‘rubygems’
require ‘ruby-prof’
RubyProf.start

This doesn’t:

require ‘rubygems’
autoload(:RubyProf, ‘ruby-prof’)
RubyProf.start #=> LoadError: no such file to load - ruby-prof

What am I doing wrong? I’d like to “weak-link” certain gems so that my
users don’t have to install a gem unless they encounter the part of my
code that actually uses it. Thx - m.