System.Configuration.ConfigurationErrorsException no WinForm

estou recebendo um

'ConnectionString' lançou uma exceção do tipo 'System.Configuration.ConfigurationErrorsException'

cada vez que tento executar meu WinForm.
Alguma idéia de como resolvê-lo?

Este é o meu código C#:

public class DB
{
    /// <summary>
    /// 
    /// </summary>
    public static string ConnectionString
    {
        get
        {
            string connStr = ConfigurationManager.ConnectionStrings["Stoica Mihai"].ConnectionString;

            OracleConnectionStringBuilder sb = new OracleConnectionStringBuilder(connStr);
            //sb.ApplicationName = ApplicationName ?? sb.ApplicationName;
            //sb.ConnectTimeout = (ConnectionTimeout > 0) ? ConnectionTimeout : sb.ConnectTimeout;

            return sb.ToString();
        }
    }
    /// <summary>
    /// Returns an opened connection to the database
    /// </summary>
    /// <returns></returns>
    public static OracleConnection GetSqlConnection()
    {
        OracleConnection conn = new OracleConnection(ConnectionString);
        conn.Open();
        return conn;

    }
    /// <summary>
    /// Override the connection timeout
    /// </summary>
    public static int ConnectionTimeout { get; set; }
    /// <summary>
    /// Used to override the name of the application
    /// </summary>
    public static string ApplicationName { get; set; }
}