Hey there Winever and Matt,
I have some experience with SKILL/Ocean scripts and my cases are similar as Matt stated. When running Monte Carlo or some Corners the circuit reaches some point that it does not have a valid value and it returns nil. If you pass nil to some other function it might throw an error.
You may try to use the errset() function (read more at Skill Language User Guide, chapter 9 Advanced Topics at Error Handling section).
As function results are stored into a list. I'll take the first element using car(). This way, you can do something like this:
bw = car( errset( bandwidth(VF("/VOUTP") 3 "low") ) )
if( bw then printf( " ok " ) else printf( "unable to evaluate expression" ) )
Hope it helps.