Why I'm getting exception: InvalidOperationException?

The Exception is on the code The code is

string cs=ConfigurationManager.ConnectionStrings["Sam"].ConnectionString;
using(SqlConnection con = new SqlConnection(cs))
{
    SqlCommand cmd = new SqlCommand("Select * from tblProduct", con);
    con.Open();
    SqlDataReader rdr = cmd.ExecuteReader();
    GridView1.DataSource = rdr;
    GridView1.DataBind();
}

So I have a one web-Config file.so I am connect with it. The code inside a web config is

<connectionStrings>
    <add name ="Sam"
    connectionString=" data source=.\\SQLExpress; database=sample; Integrated Security=true " />
</connectionStrings>

After Compiling it there is no error but when I debug it, at the run time I got

An exception of type 'System.InvalidOperationException' occurred in System.Data.dll but was not handled in user code. Additional information: Instance failure.

Why I am getting such error