Shadows smooth disappear in CG shader

Hi,

I had made a simple unlit shadow shader written in CG. The shadows work but the problem is that they don't fade out/in when approaching the shadow distance in the same way as a surface shader does. They just popin/popout. I have included the following line to make the shadow work:

#pragma multi_compile_fwdbase
#include "UnityCG.cginc"
#include "AutoLight.cginc"
LIGHTING_COORDS(x,x)
TRANSFER_VERTEX_TO_FRAGMENT(x)
LIGHT_ATTENUATION(x)

LIGHT_ATTENUATION(x) is multiplied by the final color to get the shadows.

Anyone knows how to make them behave the same way as the surface shader ones?