Can't list source using debug in ruby 1.8

Question

I have a very simple ruby program (tmp.rb) I'm trying to debug:

puts "a"
puts "b"
puts "c"
require "debug"
puts "d"
puts "d"
puts "e"

When I run it, it enters the debugger interactive mode:

a
b
c
Debug.rb
Emacs support available.

tmp.rb:5:
(rdb:1) 

However, if I try to read the source-code, I get this message:

(rdb:1) l
[0, 9] in tmp.rb
No sourcefile available for tmp.rb

Am I doing something wrong?