Building Ruby 1.9.1 pre fails on windows


I come to the step where I am told to run nmake.
Running nmake yields:

     -I./.. ./../tool/compile_prelude.rb ./../prelude.rb

‘-I.’ is not recognized as an internal or external command,
operable program or batch file.
NMAKE : fatal error U1077: ‘-I.’ : return code ‘0x1’

after the first few files have been compiled.

I would appreciate your help

regards, Oliver