Invalid string length error

[kasbah]

Somehow one of the kitspace test builds managed to trigger this error:

build/boards/github.com/kitspace-forks/index/index_piggyback/images/top-meta.png build/boards/github.com/kitspace-forks/index/index_piggyback/images/top-with-background.png

/home/travis/build/kitspace/kitspace/node_modules/xml-element-string/index.js:22

    ? '>' + children.join('') + '</' + tag + '>'

RangeError: Invalid string length
    at Array.join (<anonymous>)
    at createXmlString (/home/travis/build/kitspace/kitspace/node_modules/xml-element-string/index.js:22:22)
    at /home/travis/build/kitspace/kitspace/node_modules/pcb-stackup-core/lib/index.js:114:20
    at Array.reduce (<anonymous>)
    at pcbStackupCore (/home/travis/build/kitspace/kitspace/node_modules/pcb-stackup-core/lib/index.js:88:16)
    at finishLayer (/home/travis/build/kitspace/kitspace/node_modules/pcb-stackup/index.js:83:21)
    at PlotterToSvg.finishConversion (/home/travis/build/kitspace/kitspace/node_modules/gerber-to-svg/index.js:101:14)
    at Object.onceWrapper (events.js:421:28)
    at PlotterToSvg.emit (events.js:315:20)
    at endReadableNT (/home/travis/build/kitspace/kitspace/node_modules/gerber-to-svg/node_modules/readable-stream/lib/_stream_readable.js:1094:12)

https://travis-ci.org/github/kitspace/kitspace/builds/722689771#L6965

If it persists I will try and reproduce it on my machine.