CreateWindow возвращает NULL

#include <windows.h>
 
LRESULT CALLBACK WindowProcess(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam);
 
HINSTANCE hInstance;
 
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR CmdLine, int CmdShow)
{
    ::hInstance = hInstance;
 
    char ClassName[] = "Gradient";
    MSG Msg;
    WNDCLASSEX WndClass;
    HWND hMainWnd;
 
    WndClass.cbSize        = sizeof(WNDCLASSEX);
    WndClass.style         = 0;
    WndClass.lpfnWndProc   = WindowProcess;
    WndClass.cbClsExtra    = 0;
    WndClass.cbWndExtra    = 0;
    WndClass.hInstance     = hInstance;
    WndClass.hIcon         = LoadIcon(NULL, IDI_WINLOGO);
    WndClass.hCursor       = LoadCursor(NULL, IDC_ARROW);
    WndClass.hbrBackground = CreateSolidBrush(RGB(207,211,190));
    WndClass.lpszMenuName  = NULL;
    WndClass.lpszClassName = ClassName;
    WndClass.hIconSm       = LoadIcon(NULL, IDI_WINLOGO);
 
    if(!RegisterClassEx(&WndClass))
    {
        MessageBox(NULL, "Error with register wnd class", "Fatal error", MB_OK | MB_ICONERROR);
        return 0;
    }
 
    hMainWnd = CreateWindow(ClassName, "Gradient", WS_OVERLAPPED | WS_CAPTION, CW_USEDEFAULT, 0, 800, 600, NULL, NULL, hInstance, NULL);
 
    if(!hMainWnd)
    {
        MessageBox(NULL, "Cannot create window", "Fatal error", MB_OK | MB_ICONERROR);
        return 0;
    }
 
    ShowWindow(hMainWnd, CmdShow);
 
    while(GetMessage(&Msg, NULL, 0, 0))
        DispatchMessage(&Msg);
 
    return Msg.wParam;
}
 
LRESULT CALLBACK WindowProcess(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam)
{
    switch(Msg)
    {
    case WM_CLOSE:
        DestroyWindow(hWnd);
        break;
 
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
 
    default:
        DefWindowProc(hWnd, Msg, wParam, lParam);
    }
    return 0;
}

DefWindowProc(hWnd, Msg, wParam, lParam);

return (DefWindowProc(hWnd, Msg, wParam, lParam));