wrong behavior of isoWeekday()

[Hugoist]

Describe the bug

dayjs('random srting').isoWeekday()
returns 7

Expected behavior
It should returns NaN (as it do dayjs('random srting').day())

Information

• Day.js Version [v1.10.7]
• OS: [linux Mint 20.2]
• Browser [chrome 92.0.4515.159]
• Time zone: [GMT+0300 (Eastern European Summer Time))]

[mandaputtra]

This behaviour is intended,

// the implementation
  proto.isoWeekday = function(week) {
    if (!this.$utils().u(week)) {
      return this.day(this.day() % 7 ? week : week - 7)
    }
    return this.day() || 7
  }

[Hugoist]

Why it is 7?