Working with grayscale png images

How to make it work with grayscale png images?

I'm having trouble trying to merge a text layer with a grayscale png image.
Everything works fine if the image is RGB and the result is the same RGB image with a merged text on top.

But if the image is grayscale, the resulting file after the `save()` function is an empty file.

Thanks in advance, any suggestions are greatly appreciated.