As SKU is an optional reference, >'SKU' may or may not produce a node holding a value for Model. You need to handle the possibility that no value exists for Model. You can do that by specifying a derivation in your application model, which you can then refer to in your implementation.alan:
'Model with fallback': text = switch >'SKU' (
| node as $ => $ .'Model'
| none => "<no SKU available>"
)