Rubygems.rb:87: uninitialized constant Gem::Mutex (NameError)

Hi folks,
I’m trying out code examples from Pickaxe 2nd ed. using the regular Ruby
interpreter running on WinXP, and editing/running via RDT in Eclipse.
Examples from the chapter on multithreading are giving me this error
message – though after displaying it, the program seems to run fine.
Any thoughts on this?

It would help if you actually posted ur code