An accumulation of the income of trust property may be directed in the will or other writing creating a spendthrift trust, for the benefit of one or more beneficiaries, to commence within the time permitted for the vesting of future interests and not to extend beyond the period limiting the time within which the absolute power of alienation of property may be suspended. If the direction is for a longer term than is permitted by law, it is void only as to the excess time, whether the direction be separable from the other clauses in the trust or not, and in such cases of invalidity the income may be paid and distributed to the next succeeding beneficiary in interest.