Any policy containing a provision for a deferred annuity on the life of the insured only, unless paid for by a single premium, shall provide that in the event of the nonpayment of any premium after three full years’ premium shall have been paid, the annuity shall automatically become converted into a paid-up annuity for such proportion of the original annuity as the number of completed years’ premiums paid bears to the total number of premiums required under the contract.