StretchBlt only works when nHeightDest is negative - c++

Question

I'm trying to use StretchBlt in order to copy pixels from a memory hdc to the window hdc.
The memory hdc gets the image from an invisible window which renders a stream using openGL.
Here's my code:
BITMAPINFOHEADER createBitmapHeader(int width, int height) {
BITMAPINFOHEADER header;
header.biSize = sizeof(BITMAPINFOHEADER);
header.biWidth = width;
header.biHeight = height;
header.biPlanes = 1;
header.biBitCount = 32;
header.biCompression = BI_RGB;
header.biSizeImage = 0;
header.biXPelsPerMeter = 0;
header.biYPelsPerMeter = 0;
header.biClrUsed = 0;
header.biClrImportant = 0;
return header;
}
...
HDC memoryHdc = CreateCompatibleDC(windowHdc);
BITMAPINFO bitmapInfo;
bitmapInfo.bmiHeader = createBitmapHeader(targetDimensions.width, targetDimensions.height);
HBITMAP bitmap = CreateDIBitmap(windowHdc, &bitmapInfo.bmiHeader, CBM_INIT, offscreenBuffer, &bitmapInfo, DIB_RGB_COLORS);
SelectObject(memoryHdc, bitmap);
DeleteObject(bitmap);
SetStretchBltMode(windowHdc, COLORONCOLOR);
StretchBlt(windowHdc,
targetDimensions.x, targetDimensions.y,
targetDimensions.width, -targetDimensions.height,
memoryHdc,
sourceDimensions.x, sourceDimensions.y,
sourceDimensions.width, sourceDimensions.height,
SRCCOPY);
DeleteDC(memoryHdc);
Where windowHdc is the hdc of the window to which I want the StretchBlt to copy the pixels to, and offscreenBuffer is a void* to the pixels copied from the offscreen window in which the openGL is rendering.
This code works great, except that the image is upside down and I want it vertically flipped.
I know that this happens because:
If nHeightSrc and nHeightDest have different signs, the function
creates a mirror image of the bitmap along the y-axis
But when I remove the minus sign and both target and source heights are the same then I see no image in the window.
Just to check, I tried to put the minus on the sourceDimensions.height but that also results in no image, and the same if I try to negate the widths (both target and source).
Any idea why?
Thanks.