INDEX and MATCH problem

Hi there, I'm trying to update a template for work that deals with order release. The step I'm currently having problems with is trying to index the current backlog to the previous days backlog using two cells (Sale Order Number and Line Number) as the index. The old template I'm working off was used in Excell 2003 format.

I've attached the worksheet I'm trying to update. My problem is the formula in cell L2 of the BLQ tab, and the forumla looks like this:

=INDEX('NEW BLQ'!$I$2:$I$1000,MATCH(A2&B2,'NEW BLQ'!$D$2:$D$1000&'NEW BLQ'!$E$2:E$1000,0))

Basically I want to index (in this case) the request date on the new backlog (New BLQ tab, column I), against the request date on the old backlog (unsurprisingly called the Old BLQ tab) using Cell A2 (Sales order) and B2 (Line number) to match the values. This means when I paste the order backlog information into columns A to E, it will auto-populate from columns L to AB using a similar reference formula.

I copied over the formula from the old worksheet and corrected the tabs, but instead of returning a date I get #VALUE.

I don't know if it's significant, but the formula on the older worksheet has additional parenthesis which I'm not sure the significance of.

{=INDEX('NEW BLQ'!$I$2:$I$1000,MATCH(A2&B2,'NEW BLQ'!$D$2:$D$1000&'NEW BLQ'!$E$2:E$1000,0))}

I appreciate any help - thanks!