Same rules go for the mask as the heightmap, but instead of the picture being in grayscale, this should be in RGB.
Same size on both sides like 512x512, 1 layer only, so make sure you flatten the image, and that it's an .png image, RGB mode.
If you know you did all this, try making your mask picture into a .bmp or .jpeg, load that image, and then re-convert it back into a .png again. If you do this correctly it might help.
If that fails to work you should probably recheck things like where you place the image, make sure there's no copies of it, writing the name of the image correctly? aye.. things like that.