getcell() returns null and I don't know why … :(

Can someone help me with this piece of code I just wrote …?

p_array is null line 25

This is the line of the if()

I even filled (dumbly) the p_array slots… but no way.

autowatch = 1;
outlets = 2;

var w = 15;

var outmatrix = new JitterMatrix(3,"float32",w,1);
outmatrix.setall(0);

var scalematrix = new JitterMatrix(3,"float32",w,1);
scalematrix.setall(1.);
outlet(1,"jit_matrix",scalematrix.name);

var p_array = new Array(3);
p_array[0] = 0;
p_array[1] = 0;
p_array[2] = 0;

function bang()
{
for (var i = 0 ; i < w ; i++)
{
p_array = outmatrix.getcell(i);

if (p_array[0] < 20) p_array[0] += Math.random();
else p_array[0] = 0;

outmatrix.setcell3d(i,p_array[0],p_array[1],p_array[2]);
}

outlet(0,"jit_matrix",outmatrix.name);
}