keyword intergratedsecurity not supported

Hi I get the following error. when I try to connect to the SQl Server. The keyword IntergratedSecrity isn't supported. What did I do wrong ?

here is my class and method method to create a connectionstring.

using System.Configuration;
using System.Data.SqlClient;
namespace WindowsFormsApplication1.DataAccessLayer
{
class DatabaseLayer
{
public static SqlConnection OpenConnection()
{
var connection = new SqlConnection(ConfigurationManager.ConnectionStrings["customers"].ConnectionString);
connection.Open();
return connection;
}
}
}

and here is my ap.config

"1.0" encoding="utf-8" ?>
"customers" connectionString="Data Source=\\MAC-CASPER\SQLSERVERCASPER; Initial Catalog=Customer_xxxxx; IntegratedSecurity=True" providerName="System.Data.SqlClient" />