TypeError: Cannot read property 'mtime' of undefined

Every now and then I get following exception:
```
TypeError: Cannot read property 'mtime' of undefined
     at /home/labuser/nodejs/weather-express/node_modules/less-middleware/lib/middleware.js:291:59
     at FSReqWrap.oncomplete (fs.js:123:15)
```

I'm using less-middleware version 2.2.0 and NodeJs 6.

I don't know what causes this. Any hint would be welcome.