Passing callback functions to C library throwing Access violation

First and foremost, I'm a C# developer working on a project that was thrust on me and I'm feeling very outside of my comfort zone.

That being said, I'm currently attempting to pass 2 callback functions to a C library. I have the functions defined as free functions in my C++ console application.

When I run the application I'm receiving the following extremely unhelpful error at the point that I attempt to pass the functions to the library.

Exception thrown at 0x0000000000000000 in YourApp.exe: 0xC0000005: Access violation executing location 0x0000000000000000

The callbacks are defined in the header file for the accompanying C source file which I have listed below. Note, I did not write the following header.

header.h

typedef void( __stdcall * Callback1 )( void *, int, unsigned int, int, int, int, int );
typedef void( __stdcall * Callback2 )( void *, int, int, unsigned int, unsigned int );
__declspec(dllimport) int open( Callback1 cb1, Callback2 cb2 );

From my C++ console application I first include the C header.

extern "C"
{
    #include "header.h"
}

I then declare the 2 free functions I'm attempting to pass as parameters to the open function exposed through the header.h file.

void __stdcall callback1(
    void *data,
    int count,
    unsigned int colorId,
    int flag,
    int pageNumber,
    int width,
    int height )
{
    if ( count <= 0 ) return;
    std::cout << "callback1" << std::endl;
}

void __stdcall callback2(
    void *data,
    int count,
    int offset,
    unsigned int colorId,
    unsigned int side )
{
    if ( count <= 0 ) return;
    std::cout << "callback2" << std::endl;
}

Within main I then attempt to pass the above functions as parameters to the open function.

int main( int argc, char *argv[ ], char *envp[ ]  )
{
    // This is the line where the error is thrown.
    open( callback1, callback2 );  
}

I'm currently using VS2015. I'm unsure how to proceed as I find that error extremely unhelpful.

I'm currently doing everything in the code file that main is defined in. So the functions I have posted above are not methods of a class.