Opaque error for invalid CSS after upgrade: TypeError: Cannot read property '0' of undefined

Created by: buildbreakdo

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 (closed) (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):

Add invalid CSS to any .css file, like:

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

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).