A grantee of real estate which is subject to ground rent or bound by mortgage or other encumbrance, shall not be personally liable for the payment of such ground rent, mortgage or other encumbrance, unless he shall, by an agreement in writing, have expressly assumed a personal liability therefor, or there shall be express words in the deed of conveyance stating that the grant is made on condition of the grantee assuming such personal liability: Provided, That the use of the words “under and subject to the payment of such ground rent, mortgage or other encumbrance,” shall not alone be so construed as to make such grantee personally liable as aforesaid.