How to get Sumproduct to ignore a criteria if it is blank?

Hi,

I have three criteria for a sumproduct function. One of the criteria is sometimes blank and I would like to then have Sumproduct ignore it and only use the other two criteria.

I tried inserting an IF function: if the third criteria cell is blank, substitute the range for a "1". But that returns an error.

=SUMPRODUCT((Sheet1!$A$43:$A$211=$A16)*(Sheet1!$B$43:$B$211=$B16)*(IF(ISBLANK($C16),1,Sheet1!$C43:$C$211=$C16)*(Sheet1!$P$43:$R$211))

Is this possible in one Sumproduct formula or do I need two Sumproduct formulas?

Lawrence