Baked normal map disappear around corners

Hello,

  I ran into a strange problem yesterday. I tried to bake a high-poly on a low, and it was fine, until I noticed that there was no normal map around the corners of my mesh, but the normal was ok a bit further. As you can see in the picture below, on my mesh, the ambient occlusion baked well, we see a thin dark line, but the normal map doesn't appear at all, it only does when we go a more in the middle of the face.

  So, I tried on a simple cube, to see. I didn't touch the UVs, it's a basic cub unwrap, and the high-poly cube is the same size than the low-poly. Here's my high-poly :

 And that's what I have on my low-poly after baking :

 As you see, the normal map in the middle is fine, but there's nothing around the corners, only the ambient occlusion. If I check on the normal map :

 And here are my baking settings

 I tried to change different parameters, like the ray distance, and all, but it fixed nothing. It might be a totally dumb thing, but I can't see it x)

  Thanks in advance for any answer !