Error Cannot read property 'assetGraph' of undefined

[ehmicky]

The following error happened in a build:

a promise was rejected but not handled: TypeError: Cannot read property 'assetGraph' of undefined 
    /opt/buildhome/.netlify-build-plugins/node_modules/assetgraph/lib/assets/Asset.js:1487:16 makeRelativeToCwdIfPossible
    /opt/buildhome/.netlify-build-plugins/node_modules/assetgraph/lib/assets/Asset.js:1495:9 Asset.get urlOrDescription [as urlOrDescription]
    /opt/buildhome/.netlify-build-plugins/node_modules/hyperlink/lib/index.js:440:34 processAsset
    /opt/buildhome/.netlify-build-plugins/node_modules/hyperlink/lib/index.js:715:9 proceed
    /opt/buildhome/.netlify-build-plugins/node_modules/hyperlink/lib/index.js:717:11 

This is a private repository unfortunately. The build logs do not show any additional information.
Do you know what the problem might be?