Custom list field not found in formula

Hi - What a great tool Mockaroo is, only just found it today.

Got a problem with a formula.

if field('SMETS Version') == 'S2' then ' ' end

The field is definitely there, I copied and pasted the name in. I wonder if it's because it's a Custom List? Would that be the cause?
Also how do you put a blank value in, instead of a space? Not a big problem but still...

Thanks for your help,
Pete