dependencyTree only returning an empty object

I'm hoping someone can help me understand how to use this, as I seem to be getting an empty object back when using `dependencyTree`.

I have files along the lines of:
```
.
└── src/
├── file1.js (doesnt depend on anything)
├── file2.js (depends on file1.js)
└── file3.js (depends on file1.js and file2.js)
```
Above, where I state that a file depends on another, it imports the other file with `require()`.

And I'm trying to get the dependency info like so:
```js
const srcFiles = await fs.readdir('./src/');
const trees = srcFiles.map(filename => dependencyTree({ filename, directory: './src' }));
console.log(trees);
```
Which outputs:
>[ {}, {}, {} ]

Obviously I'm expecting something other than an empty object for each file. If anyone can help me understand how to use this I'd appreciate it.