Data in not displayed on checkbox in c#.net sqlserver

Question

I am trying to display the values from database. The textbox values are displayed successfully but checkbox values are not displayed, and it shows the error. i attached the error on screenshot image below

sql = "select * from repair where repairid = '" + repairid + "'";
SqlCommand cmd = new SqlCommand(sql, con);
SqlDataReader dread;
con.Open();
dread = cmd.ExecuteReader();
while (dread.Read())
{
checkBox7.CheckState = dread[6].ToString();
}