Binding data on ReportViewer

I am developing a Report using ReportViewer with ASP.NET and C#. I'm binding data from a DataSource to a Tablix object using the .RDLC design view.

Inside this Tablix, there's a field that must me filled with one field from de datasource OR another one depending of an expression.

The expression in the field I am trying to use to achieve that is:

=IIf(IsNothing(Fields!PREMIO_CALCULADO_1.Value), Fields!PREMIO_CALCULADO_1.Value,Fields!RECUPERACAO_CALCULADA_INI_1.Value)

If the Field "PREM开发者_JS百科IO_CALCULADO_1" is null, then fill it with "RECUPERACAO_CALCULADA_INI". I'm not having compilation errors but when I execute the report where this value should be there is an #error tag.

Am I doing something wrong with that expression?

Thank you very much.

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It looks like you have the return values in the wrong position.

Just add a NOT before IsNothing.