Is a Mudharabah contract valid if the profit share is specified vaguely?

Primary text

The Mudharabah contract is invalid if the profit share is stated as 'a portion of the profit,' 'partnership in profit,' 'some of the profit,' 'a share,' or 'a portion.' This is because the profit amount becomes unknown (*majhul*), and the Mudharabah is only valid upon a specified, known share.