Transparent bitblt

I have a program which can in some cases paint two or more bitmaps over one another. When there is only one bitmap and is against a white background, it paints fine, however when it paints itself ontop of another bitmap then it loses its colors.

Here is my bitmap-drawing function:

void CExView::DrawBitmap(CBitmap *obj,CBitmap *msk,int x,int y,int cx,int cy)
{
CBitmap *old;
CClientDC dc(this);
CDC mem;

mem.CreateCompatibleDC(&dc);

old = mem.SelectObject(msk);
dc.BitBlt(x,y,cx,cy,&mem,0,0,SRCINVERT);

mem.SelectObject(obj);
dc.BitBlt(x,y,cx,cy,&mem,0,0,SRCAND);
mem.SelectObject(old);

ReleaseDC(&dc);
ReleaseDC(&mem);
}




The mask has the transparent bits as black and the truecolor bits as white. The actual picture's transparent bits are white.

What am I doing wrong?

Thanks in advance.