How do I Stop light from going diagonally between tiles

I have a tileset that creates shadows, the problem is that when I place two tiles diagonally some light gets through at the corner, how do I fix this

Here is what I mean:

Not sure on a proper fix, but a temporary fix might be to add some edge tiles in the corners. They could even be transparent as long as they have some collision

— Thanks, but the player is going to be placing tiles, however I may be able to use this if I can't find a better solution