I have a dynamic Lookup field. Each record in the "Sites" form has an "AMUser", and the current user sees a filtered list based on this. The lookup is configured like this: 

1.     must  have  Site1
2.     (
3.         displayname  =  "Site"
4.         type  =  picklist
5.         values  =  Sites[AMUser == zoho.loginuser].ID
6.         displayformat = [ Site_name ]
7.         sortorder  =  ascending
8.         searchable  =  true
9.         width  =  206px
10.     )

However, I'd like to nest this in an If statement so that if the current user has not been allocated any Sites, they see the full list. Something along the lines of:

1. CountList = Sites[AMUser == zoho.loginuser].count()
2. IF Countlist = 0 THEN
3. {
4. values = Sites.ID
5. }
6. Else
7. {
8. values = Sites[AMUser == zoho.loginuser].ID
9. }

I'm not sure how to code this in the Form Definition and keep getting 'null' errors. Can anyone point me in the right direction?