emacs not saving files

I've a file $HOME/.rsync-EXCLUDE (listing files to exclude from my rsync based backup scripts) but when I try and save it, emacs refuses with message
File /Users/mkbane/RCS/.rsync-EXCLUDE is not an RCS master file

ANy thoughts? (I need to check this holds for all my emacs... this particular instance was on a different Linux dist since my Debian box is offline this mortning)

many thanks

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