Row Visibility Expression

I have a table that returns multiple rows.  I want to display just the first row.  I have sorting set so that row 1 is the only data that I need displayed but I am having difficulty "hiding" the subsequent rows.

I attempted the following:

1. Add a column and insert a rowcount expression.

2. Then use the following row visibility expression as hidden:

=IIf(RowNumber("TablixName")>1),TRUE,FALSE

That gave me the following error:

The Visibility.Hidden expression for the tablix ‘CaseNamebySession’ contains an error: [BC30455] Argument not specified for parameter 'FalsePart' of 'Public Function IIf(Expression As Boolean, TruePart As Object, FalsePart As Object) As Object'.

Any suggestions would be appreciated.