BackstopJS: BackstopJS test execution failing: "Error: Stream not writable"

So I’ve just started tinkering with BackstopJS. I setup some basic tests and was trying to work it into my normal build process in gulp.

For some reason though, the script just stopped executing correctly. The test worked fine several times earlier today, I came back later, made some CSS changes in my project, ran another test, and received this error:

Bitmap file generation completed.
COMMAND | Executing core for `report`
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: Stream not writable
    at ChunkStream.write (/Users/*****/Sites/*****/node_modules/pngjs/lib/chunkstream.js:46:24)
    at PNG.write (/Users/*****/Sites/*****/node_modules/pngjs/lib/png.js:92:16)
    at ReadStream.ondata (_stream_readable.js:525:20)
    at emitOne (events.js:77:13)
    at ReadStream.emit (events.js:169:7)
    at readableAddChunk (_stream_readable.js:146:16)
    at ReadStream.Readable.push (_stream_readable.js:110:10)
    at onread (fs.js:1738:12)
    at FSReqWrap.wrapper [as oncomplete] (fs.js:576:17)

For comparison, here is the same output from earlier today:

Bitmap file generation completed.
COMMAND | Executing core for `report`
compare | ERROR { size: isDifferent, content: 40.95%, threshold: 0.1% }: ***** *****_0_document_0_phone.png
   See: /Users/*****/Sites/*****/backstop_data/bitmaps_test/20161115-135709/failed_diff_*****_0_document_0_phone.png
compare | OK: ***** *****_0_document_0_phone.png
compare | OK: ***** *****_0_document_0_phone.png
compare | ERROR { size: ok, content: 1.06%, threshold: 0.1% }: ***** *****_0_document_1_tablet_v.png
   See: /Users/*****/Sites/*****/backstop_data/bitmaps_test/20161115-135709/failed_diff_*****_0_document_1_tablet_v.png
compare | OK: ***** *****_0_document_1_tablet_v.png
compare | OK: ***** *****_0_document_1_tablet_v.png
compare | ERROR { size: ok, content: 0.60%, threshold: 0.1% }: ***** *****_0_document_2_tablet_h.png
   See: /Users/*****/Sites/*****/backstop_data/bitmaps_test/20161115-135709/failed_diff_*****_0_document_2_tablet_h.png
compare | OK: ***** *****_0_document_2_tablet_h.png
compare | OK: ***** *****_0_document_2_tablet_h.png
 report |
          Test completed...
 report | 6 Passed
 report | 3 Failed

I tried deleting the html_report directory and rerunning the script. That didn’t work. I tried deleting bitmaps_reference and bitmaps_test. It could recreate the images, but the test failed in the same fashion.

There’s also nothing fancy about the backstop.json file. I think it’s just adapted from one of the demos:

{
  "id": "*****",
  "viewports": [
    {
      "name": "phone",
      "width": 320,
      "height": 480
    },
    {
      "name": "tablet_v",
      "width": 568,
      "height": 1024
    },
    {
      "name": "tablet_h",
      "width": 1024,
      "height": 768
    }
  ],
  "scenarios": [
    {
      "label": "*****",
      "url": "*****/*****/index.html",
      "readyEvent": "backstopjs_ready",
      "misMatchThreshold" : 0.1,
      "onBeforeScript": "onBefore.js",
      "onReadyScript": "onReady.js"
    },
    {
      "label": "*****",
      "url": "*****/*****/index.html",
      "readyEvent": "backstopjs_ready",
      "misMatchThreshold" : 0.1,
      "onBeforeScript": "onBefore.js",
      "onReadyScript": "onReady.js"
    },
    {
      "label": "*****",
      "url": "*****/*****/index.html",
      "readyEvent": "backstopjs_ready",
      "misMatchThreshold" : 0.1,
      "onBeforeScript": "onBefore.js",
      "onReadyScript": "onReady.js"
    }
  ],
  "paths": {
    "bitmaps_reference": "backstop_data/bitmaps_reference",
    "bitmaps_test": "backstop_data/bitmaps_test",
    "casper_scripts": "backstop_data/casper_scripts",
    "html_report": "backstop_data/html_report",
    "ci_report": "backstop_data/ci_report"
  },
  "casperFlags": [],
  "engine": "phantomjs",
  "report": ["browser"],
  "debug": false
}

Does anyone have any ideas?