SUMIF with dates

Hi all
I'm doing a formula using SUMIF
When the criteria in the formula is a date, it returns 0 Why ? I don't know
My formula is
Code:
=SUMIF(DEBTS!$A$5:$C$150;D2;DEBTS!$C$5:$C$150)
D2 is a cell that have a date
this formula results 0 value
even though it returns correct values if D2 is not a date
Hope I'm clear enough
using office Excel 2003
waiting

Either your dates in date range or the criteria range is not a proper date value.

what the following gives ?

=isnumber(d2)

and

=isnumber(DEBTS!$A$5)