'Procent.Data.Master.connectionString' is inaccessible due to its protection level

Vad gör jag för fel?

namespace Procent.Data
{
    public class Master
    {
        private string connectionString;
        public string ConnectionString
        {
            get
            {
                return connectionString;
            }
            set
            {
                connectionString = value;
            }
        }
    }

    public class SqlServer : Master
    {
        public SqlConnection GetConn()
        {
            SqlConnection myConnection = new SqlConnection();
            myConnection.ConnectionString = [B]connectionString[/B];
            return myConnection;
        }
    }
}