Struct and array

Question

hi,

i've this code

public class a

{

sa sa1 = new sa(0);

public void test ()

{

uint i, j;

for (i = 0; i <= 1; i += 1) {

for (j = 0; j <= 13; j += 1) {

sa1.va[i].vb[j] = false;

}

}

}

public struct sa

{

public sb[] va;

// other variable

public sa(uint i): this()

{

va = new sb[2];

}

}

public struct sb

{

public bool[] vb;

// other variable

public sb(uint i): this()

{

vb = new bool[14];

}

}

}

When i try to run void test i receive this error: ...Object reference not set to an instance of an object"

where am I doing wrong?