I can't help but laugh when I hear my coach stuttering.

I can't help but laugh when I hear my coach stuttering. I have practiced table tennis since 2014 I think. And my coach there stutters and sometimes it sounds really funny and I laugh and then I feel like the biggest asshole in the world. My coach is also a family friend, he knows my father and he's like the kindest person ever. Can someone tell me how to hold my laugh or something because I feel really bad even as I am writing this. I can imagine how bad it feels to be laughed at when you stutter. I don't want to make people feel this way even more because they're not doing it on purpose.