SlimerJs incompatible with Webpack as devDependency

With the engine set to `slimerjs` in the standard backstop.json (from genConfig), and the following dependencies in package.json,

```
"optionalDependencies": {
"backstopjs": "^2.3.5",
"slimerjs": "^0.10.2"
},
"devDependencies": {
"webpack": "^1.14.0"
}
```

running `npm run reference` from within the `node_modules` folder does not produce any output images.

Terminal output is
```
CasperJS: CREATING NEW REFERENCE FILES
CasperJS: onBefore.js has run for phone.
CasperJS: onBefore.js has run for tablet_v.
CasperJS: onBefore.js has run for tablet_h.
CasperJS: onReady.js has run for: phone
CasperJS:
CasperJS: onReady.js has run for: tablet_v
CasperJS: onReady.js has run for: tablet_h
CasperJS: Comparison config file updated.
```

And with debugging switched on
```
Testing script failed with code: 1

An unexpected error occured. You may want to try setting the debug option to `true` in your config file.
COMMAND | Command `reference` ended with an error
COMMAND | Error: An unexpected error occured. You may want to try setting the debug option to `true` in your config file.`
```

Removing Webpack from the dependencies in this minimal test scenario, or switching the engine to PhantomJs will give me screengrabs again, but these are requirements for the project in which I am installing Backstop.

Any suggestions as to how I can debug further? Thanks!