§ 421. Security interest in other property. Except as provided in section four hundred ten of this chapter, no retail instalment contract or obligation, or any other agreement executed in connection therewith, may provide for the creation of a security interest in any real or personal property, other than the goods which are the subject matter of the retail instalment sale, to secure the payment of the time sale price under such contract or obligation or any part thereof; provided, however, that the prohibition of this section shall not be applicable to the taking of a mortgage upon real property to secure the payment of the time sale price of a retail instalment obligation, if the transaction which gave rise to such obligation was the furnishing of goods or services for repairs, alterations or improvements upon, or in connection with buildings upon, such real property. Any such prohibited provision shall be void but shall not otherwise affect the validity of the contract or obligation.