Switch the crosstab to SQL View (View menu.)
That start of the SQL statement reads something like this:
TRANSFORM Sum(tblInvoiceDetail.Quantity) AS SumOfQuantity
Add Nz() around the expression, e.g.:
TRANSFORM Nz(Sum(tblInvoiceDetail.Quantity),0) AS SumOfQuantity
JET probably won't understand the data type corrrectly after that, so I
suggest you use CLng(), CDbl() or CCur() around that expression - depending
if your data is Long, Double, or Currency. Example:
TRANSFORM CCur(Nz(Sum(tblInvoiceDetail.Quantity),0)) AS SumOfQuantity
More information in:
Crosstab query techniques
at:
http://allenbrowne.com/ser-67.html