Debugger not available

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?