caller_for_all_threads fails to work in a multithread program using sleep conditions

test_caller.rb

#!/usr/bin/env ruby

Thread.new do
  sleep(15)
  puts "dumping stacks"
  puts caller_for_all_threads.inspect
  puts "done."
end


while true do
  sleep(10)
  puts "Sleeping thread"
end

# This should dump the stacks after 15s of runtime.