Loading module such as irb1.9 --readline is not possible in lenny. Package: irb1.9

[paul <ppalmes@gmail.com>]

at irb1.9 prompt, typing conf produces 

NomethodError: undefined method `sub' for :@ap_name:Symbol
        from /usr/lib/ruby/1.9.0/irb/context.rb:236:in `block in inspect'
        from /usr/lib/ruby/1.9.0/irb/context.rb:235:in `each'
        from /usr/lib/ruby/1.9.0/irb/context.rb:235:in `inspect'
        from /usr/lib/ruby/1.9.0/irb.rb:302:in `output_value'
        from /usr/lib/ruby/1.9.0/irb.rb:150:in `block (2 levels) in eval_input'
        from /usr/lib/ruby/1.9.0/irb.rb:263:in `signal_status'
        from /usr/lib/ruby/1.9.0/irb.rb:146:in `block in eval_input'
        from /usr/lib/ruby/1.9.0/irb/ruby-lex.rb:243:in `block (2 levels) in each_top_level_statement'
        from /usr/lib/ruby/1.9.0/irb/ruby-lex.rb:230:in `loop'
        from /usr/lib/ruby/1.9.0/irb/ruby-lex.rb:230:in `block in each_top_level_statement'
        from /usr/lib/ruby/1.9.0/irb/ruby-lex.rb:229:in `catch'
        from /usr/lib/ruby/1.9.0/irb/ruby-lex.rb:229:in `each_top_level_statement'
        from /usr/lib/ruby/1.9.0/irb.rb:145:in `eval_input'
        from /usr/lib/ruby/1.9.0/irb.rb:69:in `block in start'
        from /usr/lib/ruby/1.9.0/irb.rb:68:in `catch'
        from /usr/lib/ruby/1.9.0/irb.rb:68:in `start'
  from /usr/bin/irb1.9:12:in `<main>'Maybe IRB bug!!

-- System Information:
Debian Release: 5.0.3
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: i386 (i686)

Kernel: Linux 2.6.26-2-686 (SMP w/2 CPU cores)
Locale: LANG=en_SG.UTF-8, LC_CTYPE=en_SG.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages irb1.9 depends on:
ii  libreadline-ruby1.9      1.9.0.2-9lenny1 Readline interface for Ruby 1.9
pn  ruby1.9                  <none>          (no description available)