C++ – extern “C” DLL: Debug is OK, Release throws Error C2059

I've got a DLL that I've created as a C++ Win32 application. To prevent name mangling in my DLL, I have used the EXPORT definition defined below:

#ifndef EXPORT
#define EXPORT extern "C" __declspec(dllexport)
#endif

EXPORT int _stdcall SteadyFor(double Par[], double Inlet[], double Outlet[]);

To get this code to compile, I had to go into the project's Properties and set the C/C++ Calling Convention to __stdcall (/Gz) and set Compile As to Compile as C++ Code (/TP).

This worked in Debug mode, but Release mode is throwing error C2059: syntax error: 'string' on all of my EXPORT functions – even though I have configured the Release mode settings to be the same as the Debug settings.

How do I get Release Mode to compile?

Regards,
~Joe
(Developing under Visual Studio 2008 Professional)

EDIT:
A lot of comments about my #define, which does not appear to be causing any problems.

To eliminate the confusion, my header file has been rewritten as follows:

#ifndef coilmodel_h
#define coilmodel_h

extern "C" __declspec(dllexport) int _stdcall steadyFor(double Par[], double Inlet[], double Outlet[], char* FileIn, char* FileOut);

#endif

That is all of it.

The error is:
Description error C2059: syntax error: 'string'
File coilmodel.h
Line 4

Again, this error only appears in Release mode, not Debug mode.
Project is a C++ Win32 DLL application.