Does not overwrite when run on VIM

[ruby3141]

my VIM is 7.4 with included 1-1689 patch. on issue #48, this plugin is expected to work interactively, but it prints

'$PROJECT_DIR/.ycm_extra_conf.py' already exists. Overwrite? [y/N]

shell returned 1

Press enter or command to continue.

(note that I'm not a native english user, and output was translated to english by me. $PROJECT_DIR is arbitrary string, not real output.)