diff --git a/autoload/fugitive.vim b/autoload/fugitive.vim
index ad432a5..84ddd4d 100644
--- a/autoload/fugitive.vim
+++ b/autoload/fugitive.vim
@@ -4736,7 +4736,7 @@ function! s:GrepSubcommand(line1, line2, range, bang, mods, options) abort
   let cmd = ['--no-pager', 'grep', '-n', '--no-color', '--full-name']
   let tree = s:Tree(dir)
   let args = a:options.subcommand_args
-  if get(args, 0, '') =~# '^-O\|--open-files-in-pager$'
+  if get(args, 0, '') =~# '^\%(-O\|--open-files-in-pager\)$'
     let args = args[1:-1]
   endif
   let name_only = s:HasOpt(args, '-l', '--files-with-matches', '--name-only', '-L', '--files-without-match')