Stuck getting fastboot to work with ember-cli-slick

I'm trying to make ember-cli-slick fastboot compliant.
It uses slick.js from node_modules.
I'm attempting to get the treeforvendor hook / funnel operation working but not having much luck.
I'm following the documentation here https://www.ember-fastboot.com/docs/addon-author-guide#third-party-dependencies
but I suspect it's targetting bower rather than npm/node_modules.
```
treeForVendor(defaultTree) {
var browserVendorLib = new Funnel('node_modules/slick-carousel/slick/slick.js');

browserVendorLib = map(browserVendorLib, (content) => `if (typeof FastBoot === 'undefined') { ${content} }`);

return new mergeTrees([defaultTree, browserVendorLib]);
},
```
This results in the error:
Build Error (broccoli-persistent-filter:Mapper)

ENOTDIR: not a directory, scandir '_projectdir_/tmp/broccoli_persistent_filtermapper-input_base_path-FOInixjr.tmp/'