I'm using the formula below to calculate the values in each row. Its not
working and I'm stummped as to why. Note that there are blank cells in the
ranges listed, whether or not that makes a difference I dont know. The sheet
is formatted as follows..

=SUMPRODUCT(--(MONTH(A1:A65536=7),--(B1:B65536=C1:C65536),B1:B65536)


A B C
7-JAN 10 10
12-JAN 5 8
6-MAR 4 6
22-MAR 11 11
3-May 7 9
17-MAY 8 8

I need it to calculate only the data in columns B & C that are equal and
then total that data from only column B that matches column C while breaking
it down by month.

So in the example above the correct value returned from the formula should
be

Jan 10
Mar 11
May 8

Help in what I'm doing wrong it appreciated.