Lord of the Nazgul

Hello everyone. There's something I've been wondering about. When the the Lord of the Nazgul tells Dernhelm(Eowyn) "No living man may hinder me!" at the Battle of the Pelennor Fields, is he just referring to humans? If not, how does he know she's not an elf? He obviously has no clue that she's a woman. He's fearlessly leading an army in the middle of a huge battle, is he just assuming that there are no elves?