It takes him a moment as she starts to giggle and then as her laughter grows he finally sighs in relief and it is like his entire body just lost a huge weight. He wobbles down to the floor and sits there with a relieved expression. It seems his worry was for nothing.

"Ha..I'm going to guess you were just teasing me. That's a good joke."

It's a terrible joke..He's so relieved everything is actually okay and he doesn't have to come up with some awkward explanation to Lan Wangji.