Debugger not available

[Ralph Shnelvar]

f:\Ralph-Rails-Apps\WebOfTrust>ruby script/server webrick --debugger
=> Booting WEBrick
=> Rails 2.3.5 application starting on http://0.0.0.0:3000
f:/Ralph-Rails-Apps/WebOfTrust/config/initializers/at_file_line_msg.rb @
6:
=> Debugger enabled
=> Call with -d to detach
=> Ctrl-C to shutdown server
[2010-01-17 07:27:14] INFO WEBrick 1.3.1
[2010-01-17 07:27:14] INFO ruby 1.8.6 (2007-09-24) [i386-mswin32]
[2010-01-17 07:27:14] INFO WEBrick::HTTPServer#start: pid=3940
port=3000

***** Debugger requested, but was not available: Start server with
--debugger to
enable *****

- - - - - - - - - - -

Huh?

If I set a debugger breakpoint at some other points in my app, the
debugger comes up ... so it's not some sort of typo. And, anyway, stdout
says the debugger was enaabled.

When is the debugger not available?