Avoid things like (in bold):
set farmArea (first [[farmArea] of myFarm] of myself) + [farmArea] of my-new-farm
let my-farm [myFarm] of myself
set farmArea [farmArea] of my-farm + [farmArea] of my-new-farm
Reason: Netlogo does not work well with of…of clauses. Its commands are instable in this condition. Once it reads as value and another it reads as a list. So, one can never know when the sintax is going to work or not.
If you have different experiences, please comment here.