I just went out and tested mine, a 1/8 W - 3/16BSF wrench is correct.
According to my crossover lists, 1/8 W measures 0.340" across the flats and 9mm measures 0.354" so a 9mm could maybe work in a pinch, as long as the nut isn't too tight. A 9mm wrench will be a looser fit than, say, an 8mm wrench on a 5/16" hex 0.014" larger vs 0.0025" larger). On top of that, wrenches are typically a hair oversized (which is why a 19mm socket fits 3/4" lug nuts, despite 19mm being 0.002" smaller than 3/4").
If this is your first time removing that nut, I'd be very concerned about rounding off the hex with a 9mm wrench.