help with list box conditional expression

[#1]

I'm trying to create a list-box that will select any clients that we've assisted vs. clients that we haven't, based on the number of contacts we've made with them. Here is what I've tried, to no avail:

=if(sum(aggr(count(distinct Contacts), ClientId)) >0, 'Assisted', 'Not Assisted')

I'm not getting anything displayed in the list-box with this expression. Any thoughts?

[tresB]

May be like this?

=Aggr(if(sum(aggr(count(distinct Contacts), ClientId)) >0, 'Assisted', 'Not Assisted') , ClientId)

Though, I have a doubt if the sum() is at all needed here.

OR, possibly you can simply try:

Aggr( If(Count( DISTINCT Contacts)>0, 'Assisted' , 'Not Assisted'), ClientId )