Can anyone tell me what is wrong with the following formula?

=SUMPRODUCT((L9:L1000="O/D")*(H9:H1000)*(M9:M1000=$C$6))

The formula seems to work Ok until I add the last part (M9:M1000=$C$6).

Cell $C$6 contains a date. Essentially I am total the items in column H
which are "O/D" on a particular date (C6).

The result I am getting is #N/A.

As usual I appreciate any help given.

Thanks,

Andrew.