Visual Studio preprocessor only works if /P is set

I'm having an awkward problem in Visual Studio 2008. I'm trying to define a string-to-enum mapping using a config header (call it param_defines.h) file which looks something like this:

DEFINE_ITEM( A, BOOLEAN )
DEFINE_ITEM( B, INT )
DEFINE_ITEM( C, INT )

And so on. This is then referenced in a second header (enums.h) file:

enum ParamType
{
    BOOLEAN = 0,
    INT
};

enum Param
{
    UNKNOWN = -1
#define DEFINE_ITEM( NAME, TYPE ) ,NAME
#include "param_defines.h"
#undef DEFINE_ITEM
};

Then in a third (source) file I'm doing this:

#include "enums.h"
std::tr1::unordered_map<std::string, int> params;
#define DEFINE_ITEM( NAME, TYPE ) params[ #NAME ] = NAME
#include "param_defines.h"
#undef DEFINE_ITEM

When I compile the source file I a load of errors like:

error C2065: 'A': undeclared identifier
error C2065: 'B': undeclared identifier
error C2065: 'C': undeclared identifier

So something is going on with the preprocessor isn't quite doing what I want it to do.

The kicker is this. I set /P so I have some way of diagnosing what's going wrong. When I do this, the file compiles successfully.