Sumif function gives wrong result on Large worksheet ?

Hi Folks ,

I am new to forum and appreciate if you could shower some expertise on this problem.

The sumif function that I used delivered 95% correct result and 5% erroneous result on a large spreadsheet. The sumif function used was :

=sumif($G$4:$AC$1127,AE,$AC$4:$AC1127)

Column G contains invoice numbers , a lot of them repetitive invoice numbers.

Column AE contains invoice numbers in numerical sequence – invoice numbers not duplicated here.

Column AC is the target range to sum. It contains amounts in monetary values.

The sumif function delivered 95% correct result and 5% erroneous result on a large spreadsheet. The erroneous results cells even sum the next column of the target range.

Thanking in advance and appreciate any help.

Best regards from Singapore.

Any experts out there to help.