What is the problem with the loop?

I am programming a shader and I am getting an array index out of bounds error. The error is at :

     LightInfo lt;
 IlluminationResult illumTotal;
 int lighttemp = lights.Length;
 for (int i = 0 ; i < lighttemp; i++) {
     // Get the light informatio from i:th light
     lt.P = lights[i].get_position(mtl); // error line**
     lt.D = lights[i].get_direction(mtl); 
      ....
     }

I have followed the code back and I can't find an error.