I'm making a CGOL game in J, here's the code to get the next state given the current one:

next =: 3 :0
  res =. ,/ ({."1 pos) r"(0 0) ({:"1 pos)
  (size,size) $ res
)

Which currently doesn't run -- "noun result required in next"

However, if I change the verb to be:

next =: 3 :0
  res =. ,/ ({."1 pos) r"(0 0) ({:"1 pos)
  NB.
  (size,size) $ res
)

Then suddenly it runs fine?? Is this just some weird edge case bug or am I actually missing something here