object cannot be cast from dbnull to other types

Question

when we run the program then show the following error

object cannot be cast from dbnull to other types 

i vahe calculate the datagraid view cell

int sum = 0;

for (int a = 0; a < dataGridView1.Rows.Count; a++)

{

sum += Convert.ToInt32(dataGridView1.Rows[a].Cells[2].Value);

}

label5.Text= sum.ToString();

}