ruby-mode / inferior ruby

Emacs' ruby-mode doesn't seem to work too well. I would be most happy if
anyone could tell me if what I see is the normal behaviour, or if there
is an error in my setup. An example, given the following code:

def pow(x)
  x * x
end

pow()

When I evaluate this twice using C-c C-e, I get the following in the in-
ferior ruby buffer:

irb(main):001:0> irb(main):002:1>   irb(main):003:1>   nil
irb(main):004:0>                    ArgumentError: wrong # of arguments(0 for 1)
        from (irb):5:in `pow'
        from (irb):5
irb(main):006:0> irb(main):007:1>   irb(main):008:1>   nil
irb(main):009:0>                    ArgumentError: wrong # of arguments(0 for 1)
        from (irb):10:in `pow'
        from (irb):10
irb(main):011:0> 

As you can see, the prompt is echoed for every line evaluated, and the
line numbers are not reset so that the second evaluation reports excep-
tions on wrong lines. This is extremely annoying.

I have tried on the following combinations of software:

emacs 20.7.2 / 21.2.1
irb 0.7.4(01/05/08) / 0.9(02/07/03)
ruby-mode 1.25.2.8 / 1.58
Debian GNU/Linux Woody/Sarge

The relevant section of my .emacs looks as follows:

(require 'ruby-mode)
(require 'rubydb)

(setq auto-mode-alist
      (cons '("\\.rb$" . ruby-mode)
            auto-mode-alist))

(setq interpreter-mode-alist (append '(("^#!.*ruby" . ruby-mode))
                                     interpreter-mode-alist))
(autoload 'run-ruby "inf-ruby" "Run an intferior Ruby process")
(autoload 'inf-ruby-keys "inf-ruby"
 "Set local key defs for inf-ruby in ruby-mode")
(add-hook 'ruby-mode-hook
          '(lambda()
             (inf-ruby-keys)))

Any help *highly* appreciated!

-- 
I've seen things you people wouldn't believe. Attack ships on fire off
the shores of the Nonestic Ocean. I watched magic blunderbusses flash
and glitter in the dark near Glindea's Palace. All those moments will
be lost in time, like tears in rain. Time to die...