Terrain won't keep lightmap.

I am having an extremely annoying problem. I have a 512*512 terrain with it’s lightmap resolution set to 4048. It takes a long time to lightmap and it comes out nice. I save the scene and play to check it out. As soon as the game starts the lightmap for the terrain disappears. Only the terrain though. The lightmap on everything else including the trees on the terrain is fine. It is very annoying and I have tried everything. Thank you for your time.

Found out what the problem was. When I clicked play the lightmap index for the terrain went to -1. Don’t know why but I put the correct lightmap number and now it works.