IF formula with a Blank Date

I keep getting errors for the following formula:

=IF((ISBLANK[Issuance Deadline]),"",IF([Letter Issued]<=[Issuance Deadline]),"Timely","Untimely")

I want to leave the cell blank if [Issuance Deadline] is blank, but then add timely or untimely if the date exists.

Thanks, Lori