stats is sometimes undefined

[ehmicky]

Some builds report the following error:

Cannot set property 'post' of undefined 
    /opt/buildhome/.netlify-build-plugins/node_modules/netlify-plugin-image-optim/index.js:37:26 
    /opt/buildhome/.netlify-build-plugins/node_modules/netlify-plugin-image-optim/index.js:33:22 onPostBuild
    src/plugins/child/run.js:18:3 async Object.run
    src/plugins/child/main.js:38:38 async handleEvent
    src/plugins/ipc.js:97:9 async process.<anonymous>

This seems to indicate that some file paths returned by imagemin() point to non-existing files. I am not quire sure how this might be happening.
The repository is here. The build logs are not showing additional information unfortunately.