Hi, the behaviour of YEAR() and MONTH() has changed, the long answer is here.
The short answer is to wrap the YEAR() and MONTH() functions in IFERROR().
=ArrayFormula(sumproduct((IFERROR(month('Sheet'!$C$4:$C$499))=A35)*(IFERROR(year('Sheet'!$C$4:$C$499))=$B$34)*('Sheet'!$Q$4:$Q$499)))
BTW you can use open-ended ranges in this situation, eg $C$4:$C instead of $C$4:$C$499.
HTH Adam |