I'm getting a different error with x86_64, but probably not protected against other bugs:

(irb):3:in `[]=': failed to allocate memory (NoMemoryError) from (irb):3:in `irb_binding' from /usr/lib/ruby/1.8/irb/workspace.rb:52:in `irb_binding' from /usr/lib/ruby/1.8/irb/workspace.rb:52

sizeof(long) is different for you, and fixnum is 63(?) bits. Try 0x7fffffffffffffff.

Yes, it was. Go look what happens in gdb when you do that.