Deny her apology in kindness
From Lord Of Utterdom
"There is nothing to apologize for!" You say with a chuckle. "It's a small thing, and we all can be forgetful at points." You shake your head with a smile. "Run along, and don't concern yourself so much."
She bows graciously, offering you a huge smile. "Of course, my Lord," she says, before she runs off down the hall to return to her work.