Unless I'm mistaken, the following voter array:

[ [ 'a', 'c', 'b', 'd', 'e' ],
  [ 'd', 'c', 'a', 'b', 'e' ],
  [ 'e', 'b', 'd', 'a', 'c' ],
  [ 'e', 'a', 'b', 'c', 'd' ],
  [ 'b', 'c', 'e', 'a', 'd' ] ]
[ [ 'c', 'e' ],
  [ 'c', 'e' ],
  [ 'e', 'c' ],
  [ 'e', 'c' ],
  [ 'c', 'e' ] ]
[ [ 'c' ], [ 'c' ], [ 'c' ], [ 'c' ], [ 'c' ] ]

Should reduce correctly to 'c', but the test insists that 'e' is correct by the kata.

As stated by some users, the kata is missing the statement "if a voter is not in any first-preference vote, then remove it from all voting ballots."