Unable to build simple 64-bit C++ app in VS2008

I'm trying to build a trivial "hello world" type program using the Windows console app template with precompiled headers, on an x64 platform targetting an x64 platform.  I get the following error:

.\x64\Debug\stdafx.obj : fatal error LNK1112: module machine type 'X86' conflicts with target machine type 'x64'

this happens despite there being no mention of x86 in my project properties.  It appears that the stdafx.cpp is being compiled for x86 regardless of what I specify.

// Test64c.cpp : Defines the entry point for the console application.
#include "stdafx.h"
int _tmain(int argc, _TCHAR* argv[])
{
    return 0;
}
 
// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
#pragma once
#include "targetver.h"
#include <tchar.h>

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