opencl bug?

__kernel void add(__constant float *a, __global float *answer)
{
    const int id_x = (int)get_global_id(0);
    int i,j;
    float sum = 0;

    for(i = 0; i < 3; i++)
    {
        for(j = 0; j < 3; j++)
        {
            //This will not work
            sum +=  a[i+j];
            /*
            This will work:
            sum += a[j*i];

            So will this:
            sum += a[j];

            And this:
            sum += a[i];
            */

        }
    }
    //Write result
    answer[id_x] = sum;
}