Error when calling dump_all about memprof HOT 1 OPEN

1.8.7 :001 > require 'memprof'
Failed to determine offset of heaps_slot->limit
Failed to determine offset of heaps_slot->slot
You won't be able to dump your heap!
 => true 
1.8.7 :002 > Memprof.dump_all "file.txt"
Memprof::Unsupported: not enough config data to dump heap
    from (irb):2:in `dump_all'
    from (irb):2
    from /home/jeff/.rvm/rubies/ree-1.8.7-2012.02/lib/ruby/site_ruby/1.8/rubygems/specification.rb:1332
1.8.7 :003 >