Problem: Aggregate Functions cannot be especified as nested aggregates-

Hi everyone.

I am making a matrix report with SQL RS 2008 R2. I added a Total Column and I modified the function as show bellow:

=Sum( 
IIf(Fields!idPregunta.Value<>"16", Fields!Resultado.Value, Last(Fields!Resultado.Value))
)

I have the next error message:

"An error occurred during local report processing. The definition of the report is invalid. The value expressionfor the textrun uses a First, Last or Previous aggregate in an outer aggregate. These aggregate functions cannot be specified as nested aggregates."

What alternativa can I use to solve this problem? This is a dynamic report.

Thanks.