Visual Studio 2012 won't let me use include guards anymore

I used to be able to declare include guards like:

#ifndef DISPLAY_H
#define DISPLAY_H

But recently my Visual Studio seems to just think I'm trying to declare an if:

fatal error C1070: mismatched #if/#endif pair in file 'c:\graphics\opengltest\display.h'

Anyone know what's going on here? Seems like it could just be compiler settings or something but I don't think I changed anything from VS initial install that would have effected this, #pragma once works still but it isn't my preference.