=sumproduct({1},{0}) produces 0 as expected.
=sumproduct({1},{""}) does the same.
=sumproduct({1},{a1}) where a1 is "" does the same.
=sumproduct({1},{a1}) where a1 is =if(0,0,0) does the same
=sumproduct({1},{a1}) where a1 is =if(0,"","") produces #VALUE! for
reasons I do not understand.

Why the #VALUE! which is documented as:
"The array arguments must have the same dimensions. If they do not,
SUMPRODUCT returns the #VALUE! error value".

I am trying to take advantage of:
"SUMPRODUCT treats array entries that are not numeric as if they were
zeros".

My real data involves a function which returns non-negative numbers
including zero and "" to mean - this does not apply.

P.S. Is there any good reason why array constants can't contain function
calls. e.g. the following is not an array constant: {if(0,0,0)}

I am using Excel 2003 (11.6355.6360) SP1 on XP Professional SP2 if that
is significant.
--
Walter Briscoe