I have a project which works fine when I run npm start but fails with this error when I run npm run build or yarn build

Failed to compile.

Cannot read property 'toLowerCase' of undefined
CompileError: Begins at CSS selector .invert-color .form-checkbox input:checked,.invert-color .form-radio input:checked


npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! open-react-template@0.1.0 build: `react-scripts build`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the open-react-template@0.1.0 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
...