fatal error C1016: #if[n]def expected an identifier

Here is the code:

#ifndef 17_1OBJECTS
#include "17_1Objects.h"
#define 17_1OBJECTS

Here is the error:

fatal error C1016: #if[n]def expected an identifier



Very simple. Why? I've used #ifndef loads before and it has never given me this error.

Oh! OK it's because I can't use numbers. Got it.