String reference not set to an instance of a string. Parameter name: s

Question

private void dgv_Member_CellContentClick(object sender, DataGridViewCellEventArgs e)
{   
    string colName = dgv_Member.Columns[e.ColumnIndex].Name;
    if (colName == "colEdit")
    {
        AddNewMember add = new AddNewMember(this);
        add.btnSave.Enabled = false;
        add.btnUpdate.Enabled = true;
        add.txtAcctno.Enabled = false;
        add.txtAcctno.Text = _AccountNo;
        add.dtpBDate.Value = DateTime.Parse(_Birthdate); "this line were the error pointed"
        add.dtpMembership.Value = DateTime.Parse(_DateofMembership); "this line were the error pointed"
        add.ShowDialog();