strange behavior of report viewer 10

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SQL-Server

SQL-Server-2008

SSRS

I have a report which use two data sets (dsdtl,dsopbal) and i use lookup function to get value from dsopbal, expression is

=Lookup(Fields!FKCOA.Value,Fields!FKCOA.Value,Fields!OPBAL.Value,"dsopbal")

This report works correctly in vs report viewer, i am using SQL Server Business Intelligence Development Studio for server 2008 R2. Now when i render this report in my application it generate following exception

The Value expression for the text box ‘Textbox3’ refers to the field ‘OPBAL’. Report item expressions can only refer to fields within the current dataset scope or, if inside an aggregate, the specified dataset scope.<br />

Textbox3 contains the above expression.

Thanks