Strange vlookup issue

Hi all,

I have an issue with vlookup... I register data on a per week basis with a key in front of it (eg. 2007-04 stands for week 4 in 2007). Every week I need to calculate a figure using the previous week result. All is working well except for getting the value from the previous year when it's week 1 - it should then get the first value where the key starts with: 2006-

Example is in the attachment.

Cheers.

Steven