Using inside Heroku dyno

This is less of an issue than it is tech-support I suppose, but I can't seem to get Backstop working inside Heroku.

For sanity's sake I'm trying to run 1 scenario, capturing the entire page, with these config options:

` "engine": "chromy",
"report": ["CI"],
"cliExitOnFail": false,
"asyncCaptureLimit": 1,
"asyncCompareLimit": 1,
"debug": true,`

I can generate references and test as expected locally, but on Heroku, the following happens.
Once the app is ready and I open the app url, a testing run is started. Chromy is launched and I see the entire page html echoes in the logs, then this:

ChromyJS returned an unexpected error while attempting to capture a selector. { WaitTimeoutError: evaluate() timeout at EvaluateTimeoutError.ExtendableBuiltin (/app/node_modules/chromy/dist/error.js:44:28)

I'm stuck here. The only notable detail is that an iframe (belonging to Stripe) is parsed not long before it hangs (the last html I see is `).`

The bitmaps_test folder never gets populated with images.

As far as Heroku is concerned, I'm using 2 buildpacks, https://github.com/heroku/heroku-buildpack-google-chrome.git and heroku/nodejs.

Any help would be greatly appreciated.