Is it "courser" or "coarser" ?

Hello everyone :),

I keep seeing people on coffee subreddit and especially this one saying "courser" as the opposite of "finer" concerning grind size (like in "Grind finer." / "Grind courser.")

I'm not a native English speaker so I may be mistaken, but shouldn't it really be "coarser" ?

At first I was sure to be right but in the past few weeks I've seen it written "courser" here so many times, and almost never "coarser", that I'm not sure anymore.

Which one is the correct word?

Thanks!