¿Cuál es la sintaxis correcta aquí?

<code>If (@timestamp < (Select PromoStartTimestamp From @promo))
    RAISERROR('Code not valid until ' + (Select PromoStartTimestamp From @promo)
              ,16
              ,1);
</code>

He intentado:

<code>If (@timestamp < (Select PromoStartTimestamp From @promo))
    RAISERROR(N'Code not valid until @starttimestamp'
              ,16
              ,1
              ,(Select PromoStartTimestamp From @promo));
</code>