Problem using MAX with DATEDIFF in SSRS Report Builder expression

I am building a SSRS report, using report builder, and using an expression to  pull the last date in a group, and create number of days between it and today. 

This is the expression: =DATEDIFF(Day,Max(Fields!elig_eff_dt.Value),Today())

that returns this error:

The Value expression for the textrun ‘Textbox1.Paragraphs[0].TextRuns[0]’ contains an error: [BC30455] Argument not specified for parameter 'DateValue' of 'Public Function Day(DateValue As Date) As Integer'.

Any idea what my problem is?

Thanks