Microsoft JScript runtime error: Object doesn't support this action

I was trying to execute this code at page load and it raised this error while unloading the page.

the code which i wrote in page load is

string strCon = System.Configuration.ConfigurationManager.AppSettings["ConnectionString"].ToString();

SqlConnection sqlcon = new SqlConnection(strCon);
sqlcon.Open();

The page executes fine but while unloading it raises error.
please help. this has no jscript or javascript