CreateIconIndirect ignores icon mask

I have a 24 bpp bitmap that contains several 16x16 subpictures. The resource ID of the bitmap is IDB_TRAYICONS. I'd like to extract one of them and show it as a tray icon. My problem is that the icon created by CreateIconIndirect doesn't have a transparency mask even if I create it directly and set the hbmMask member of the ICONINFO struct. What did I do wrong?

void showTrayIcon(HINSTANCE hInst, HWND hwnd, const wchar_t *title) {
      NOTIFYICONDATA ni;
      memset(&ni, 0, sizeof(ni));
      ni.cbSize = sizeof(ni);
      ni.hWnd = hwnd;
      wcscpy_s(ni.szTip, sizeof(ni.szTip) / sizeof(*ni.szTip), title);
      ni.uCallbackMessage = WM_USER+1;
      ni.uFlags = NIF_MESSAGE + NIF_ICON + NIF_TIP;
      ni.uID = 1;

      HBITMAP icons = LoadBitmap(hInst, MAKEINTRESOURCE(IDB_TRAYICONS));
      BITMAP iconsHeader;
      GetObject(icons, sizeof(iconsHeader), &iconsHeader);

      HDC hdcIcons = CreateCompatibleDC(GetDC(0));
      SelectObject(hdcIcons, icons);

      // extract icon image (test: 16x16 icon at top left)
      HBITMAP iconImage = CreateBitmap(16, 16, 1, iconsHeader.bmBitsPixel, NULL);
      HDC hdcIconImage = CreateCompatibleDC(hdcIcons);
      SelectObject(hdcIconImage, iconImage);      
      BitBlt(hdcIconImage, 0, 0, 16, 16, hdcIcons, 0, 0, SRCCOPY);

      // create icon mask (test: checkerboard pattern)
      HBITMAP iconMask = CreateBitmap(16, 16, 1, 1, NULL);
      HDC hdcIconMask = CreateCompatibleDC(hdcIcons);
      SelectObject(hdcIconMask, iconMask);      
      for (int i=0; i<16; i++)
            for (int j=0; j<16; j++)
                  SetPixel(hdcIconMask, i, j, (i+j)%2 ? 0xFFFFFF : 0);
      
      // create icon
      ICONINFO ii;
      ii.fIcon = true;
      ii.hbmColor = iconImage;
      ii.hbmMask = iconMask;
      ii.xHotspot = 0;
      ii.yHotspot = 0;

      ni.hIcon = CreateIconIndirect(&ii);
      Shell_NotifyIcon(NIM_ADD, &ni);
      DeleteDC(hdcIcons);
      DeleteDC(hdcIconImage);
      DeleteDC(hdcIconMask);
      DeleteObject(icons);
      DeleteObject(iconImage);
      DeleteObject(iconMask);
      DestroyIcon(ni.hIcon);
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