Explain "Unexpected end of input" in combiner adventure

[marlun]

I had almost the whole solution for the combiner adventure but was getting

SyntaxError: Unexpected end of input
  at Object.parse (native)
  at Stream.write (/Users/marlun/Code/stream-adventure/combiner.js:14:18)
  at Stream.stream.write (/Users/marlun/Code/stream-adventure/node_modules/through/index.js:26:11)
  at Stream.ondata (stream.js:32:26)
  at emitOne (events.js:75:13)
  at Stream.emit (events.js:150:7)
  at drain (/Users/marlun/Code/stream-adventure/node_modules/through/index.js:36:16)
  at Stream.stream.queue.stream.push (/Users/marlun/Code/stream-adventure/node_modules/through/index.js:45:5)
  at emit (/Users/marlun/Code/stream-adventure/node_modules/split/index.js:36:14)
  at Stream.<anonymous> (/Users/marlun/Code/stream-adventure/node_modules/split/index.js:59:7)

Could someone please explain to me how one would debug the situation to figure out that what is needed is

if (line.length === 0) return;

on top of the through/write function?