using Aggregate function on Parameter value

Hi, I am using SSRS 2008 R2 and writing an expression to get the Maximum & Minimum values from a multi value parameter. This expression does not throw any error or return anything. I looked for msdn documentation to see if it said not to use Aggregate functions on Parameters but nothing as such has been documented so far. I was wondering how would I do this?

Thanks in advance.........

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Ione