SSRS - Checking whether the data is null

I've the following expression in my report.

=FormatNumber(MAX(Fields!Reading.Value, "CellReading_Reading"),3)

Now when the dataset is empty 'Fields!Reading.Value' becomes empty and finding their maximum is invalid. How can i check if the entire column is empty?

I tried the following with no luck.

=iif(IsNothing(Fields!.Reading.Value),"",FormatNumber(MAX(Fields!Reading.Value, "CellReading_Reading"),3))

But still i'm getting #Error in the report. I also checked out link and was not able to get a clue from it. I want to handle it in the report level.