Problems with merge action

I was having problems in the edit view when I use the merge command…

When I activate “set smooth” it looks like that:

How I can solve this (The black line in the center of the model)?

Thanks for your help.

select all vertices -> Ctrl+N to recalculate all the normals.

PS: normals are the directions of polygons face. Right now, it seems the center triangles are faced inwards, hence the black line.

Thanks a lot, it works well.