Do you knock before entering a single person public bathroom?

So this is one of my biggest pet peeves lol. I ALWAYS knock before entering a public bathroom (single person), regardless of if I think anyone is in there. Even if it looks like the light is off and like it's vacant, I still knock because you can never be too sure and I would hate to walk in on someone. 
However, it seems that not everyone else is this way. I have been walked in on while on the toilet more times than I can count! I always try to lock the door but sometimes the lock doesn't work or it doesn't lock properly. And people have walked in on me without even bothering to knock to see if the bathroom was in use! I know I'm probably blowing this out of proportion but it just bothers me lol. What do you ladies do?

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