The trustee is required, within 90 days after the occurrence of a default (which is known to the trustee and is continuing), with respect to the debt securities of any series (without regard to any grace period or notice requirements), to give to the holders of the debt securities of such series notice of such default; provided, however, that, except in the case of a default in the payment of the principal of (and premium, if any) or interest, or in the payment of any sinking fund installment, on any debt securities of such series, the trustee will be protected in withholding such notice if it in good faith determines that the withholding of such notice is in the interests of the holders of the debt securities of such series.