Prefer not to say, call her Mistress

"I'm sorry, Mistress, but I'd rather not talk about it."

She frowns. "You know we are far enough away to not be heard, so you may call me by my name."

You shake your head. "I am your slave, and you are my mistress. It wouldn't be right for me to call you by your name... unless you order me to, of course."

She sighs. "I would never use an order for something so frivolous. If this makes you more comfortable, go ahead."

As you continue walking, her frown deepens.