Unable to Draw With WM_NCPAINT

I am trying to paint in the non-client area, to produce a custom "skin". Though I can trap the WM_NCPAINT message, I cannot get any of the drawing functions to work. Here is a test code, to draw a small, red rectangle in the NC area. The coordinates in this test are hard-coded, to rule-out the possibility of a bad variable (the upper-left corner of the window is at screen coords 100, 100):

case WM_NCPAINT:
         {
            HDC   hdc = GetWindowDC(hwnd);
            auto ncpResult = DefWindowProc(hwnd, WM_NCPAINT, wParam, lParam);

            HRGN  rL = CreateRectRgn(401, 101, 410, 110);
            FillRgn(hdc, rL, CreateSolidBrush(0x000000FF));
            DeleteObject(rL);
            
            ReleaseDC(hwnd, hdc);
            return ncpResult;
         }

I have also tried to get the DC using this method:

HDC  hdc = GetDCEx(hwnd, (HRGN)wParam, DCX_WINDOW | DCX_INTERSECTRGN);

In addition, I have tried FillRect(), as well as the MoveToEx() and LineTo() combination. None of these have any effect.

I am using Visual Studio 2019, on Windows 7.