Concantenated SUM() problem
 

I'm trying to build a sum range statement by concantenating a range from
another sheet. The actual statement looks like this

=SUM(INDIRECT(AF31&"!O_"&INDEX($A:$A,ROW())&"_"&AL 31&":"&AF32&"!O_"&INDEX($A:$A,ROW())&"_"&AL32))

AF31 = Orders_June_2006.xls
AF32 = Orders_June_2006.xls

......in another scenario those could have different names, but lets
ignore that for now

AL31 = 12
AL31 = 22

.....these two cell values represent day codes
.....after formula evaluation it looks like this

=SUM(Orders_June_2006.xls!O_6697_12:Orders_June_20 06.xls!O_6697_22)

The cells are named cells based on product code and day. The formula
evaluation doesn't throw an error, but it never evaluates to anything
but zero. When I type the formula manually without the concatenation, I
get the number I'm supposed to. What is going wrong?? Thanks.

-Dan