SSRS2005- SUM of textbox in report footer having non-numeric values

Hi,

In my SSRS2005 report, I'm using a table. This table contains a textbox say X. The value of X can be blank string or decimal value coming from database. Now I've to display page wise sum of  X at the report footer. For this I'm using following expression in footer for a textbox Y:

=SUM( IIF(ReportItems!X.Value = "", 0, ReportItems!X.Value))

But I'm facing error "The Value expression for the textbox ‘Y’ refers to more than one report item.  An expression in a page header or footer can refer to only one report item."

How can I do this?