After upgrading from react-scripts 1.1.4 to 2.1.3 see an opaque error message when trying to build:

Error in TypeError: Cannot read property '0' of undefined 

Thought this might be related to #2069 (determined to be bug of package managers?), but after trial and error determined it was related to invalid CSS.

Reproduce (react-scripts 2.1.3):

1. Add invalid CSS to any .css file, like:

label[for=*] { margin-right: 12px; } 

2. yarn build

Output:

Error in TypeError: Cannot read property '0' of undefined 

This invalid CSS does not produce a build error in react-scripts 1.1.4, so this is something people will bump into when they upgrade and not really know what to do (e.g., claim the upgraded version is broken).