Does an agent have to be personally liable on a contract where the principal of the contract is not personally liable?

In Fridman, The Law of Agency, 7th ed. at p.237, the learned author states: Intention of Agent. ..., the general rule is that an agent will be personally liable on the contract he has made, even where he is described in the contract as acting on behalf of someone else as principle, if the intention of the parties to the contract was that the agent should be personally liable. The rule was expressed in these words by Park J. in Spittle v. Lavender: ...Where the principal is known, the agent is not liable ... though it is true that an agent may under certain circumstances render himself liable in all events. But it is not merely because he calls himself agent that he can become liable. He must so frame the undertaking as to make his additional engagement clear beyond dispute.