Which one of the following is Not an assumption of Gorden's Model of Dividend ?

Myron Gordon's Dividend Growth Model assumes that the firm is an all-equity firm with perpetual life, the internal rate of return (r) and cost of equity (ke) remain constant, the retention ratio (b) is constant once decided, and ke exceeds the growth rate (ke exceeds br). The statement that 'ke decreases with the retention rate' violates Gordon's constant-ke assumption.