bezier curve cant be illuminated?

i created a bezier curve as the ground and I put an object on the ground. Then, I want to use a light to illuminate them.
I created a spotlight to get a scene of night:

the object is illuminated and the ground around the object is illuminated too.

but I failed…

the object has been illuminated but the ground has NOT yet.

but if I create a cercle to replace the curve, everything will be right. the cercle can be illuminated.

I really dont know why the bezier curve cant be illuminated.

can u help me plz~~~