Carbon: Error in calculating previous/next months

$month = new Carbon('2016-05');
$month-> endOfMonth(); // updated
[
'month' => $month,
'monthPrev' => $month->copy()->subMonth(),
'monthNext' => $month->copy()->addMonth(),
]

Dump:

  "month" => Carbon {#233 ▼
    +"date": "2016-05-31 23:59:59.000000"
    +"timezone_type": 3
    +"timezone": "UTC"
  }
  "monthPrev" => Carbon {#241 ▼
    +"date": "2016-05-01 23:59:59.000000"
    +"timezone_type": 3
    +"timezone": "UTC"
  }
  "monthNext" => Carbon {#234 ▼
    +"date": "2016-07-01 23:59:59.000000"
    +"timezone_type": 3
    +"timezone": "UTC"
  }