S/MIME signed mail doesnt work properly without encryption

Question

When i check IsEncrypted flag on true, the message is encrypted and signed, but when IsEncrypted flag is on false, signing dont work.

       SecureMailMessage message = new SecureMailMessage()
        {
            From = new SecureMailAddress(_parameters.Sender, _parameters.Sender,  cert, cert),
            Subject = emailMessage.Subject,
            SubjectEncoding = Encoding.UTF8,
            Body = emailMessage.Body,
            BodyEncoding = Encoding.UTF8,
            IsBodyHtml = true,
            IsEncrypted = false,
            IsSigned = true
        };