What exception should I throw?

gistfile1.txt

namespace Sample1 {
    class Program {
        static void Main( string[] args ) {
            Class1 class1 = new Class1();
            Console.WriteLine( class1[ DateTime.Now.Second ].ToString() );
        }

    }

    public class Class1 {
        public int this[ int x ] {
            get {
                if ( x < 10 ) {
                    return x;
                }
                // CA1065: Do not raise exceptions in unexpected locations
                throw new IndexOutOfRangeException( "IndexOutOfRangeException" );
            }
        }
    }
}