Starting a new NextJS project, I was getting errors like
TypeError: Cannot read property 'toLowerCase' of undefined at ReactDOMServerRenderer.renderDOM (/Users/bistenes/Code/project/node_modules/react-dom/cjs/react-dom-server.node.development.js:2776:27)
Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.
which were obviously totally irrelevant to the actual problem. Turns out Reactstrap <5.0 is incompatable with React 16.
npm i --save reactstrap@next fixed it.