The operator -= is undefined for the argument type(s) double, boolean

I was developing something. But an error appears, and i cannot fix it. It says ''The operator -= is undefined for the argument type(s) double, boolean'' on my code: if (x -= 15.0D < 0.0D) { I'm just leaving out a part of my code. Anyone knows how to fix it? I cannot get it work. Thank you :)