Save Radiobuttonlist Data Into Database When They Are In Disable Position?

iam using 2 radio buttonlist controls in web application

they are

FIRST Yes No
SECOND Yes No

Initially SECOND radio button list is in disable position.

when i was click on yes of FIRST radio button list, then SECOND radio button list is enables otherwise it is in disable mode only.

here my problem is when i am inserting data into the database the disable radio button list gives error as

System.NullReferenceException: Object reference not set to an instance of an object.

code written for inserting is

protected void Button1_Click(object sender, EventArgs e)
{
SqlConnection con = new SqlConnection(@"Data Source=STS2SQLEXPRESS;Initial Catalog=STS;Integrated Security=True");
string sql;
sql = "insert into Table1(";

[Code]....

i want NULL value if i am not select SECOND radio button list