Cannot read property 'toLowerCase' of undefined

[binary64]

git clone -b react-sticky-el [email protected]:binary64/namjat.git react-sticky-el && cd react-sticky-el && yarn && yarn dev should get you http://localhost:3000/ which shows:

Cannot read property 'toLowerCase' of undefined
TypeError: Cannot read property 'toLowerCase' of undefined
    at ReactDOMServerRenderer.renderDOM (C:\p\react-sticky-el\node_modules\react-dom\cjs\react-dom-server.node.development.js:2304:28)
    at ReactDOMServerRenderer.render (C:\p\react-sticky-el\node_modules\react-dom\cjs\react-dom-server.node.development.js:2298:21)
    at ReactDOMServerRenderer.read (C:\p\react-sticky-el\node_modules\react-dom\cjs\react-dom-server.node.development.js:2234:19)
    at renderToString (C:\p\react-sticky-el\node_modules\react-dom\cjs\react-dom-server.node.development.js:2501:25)
    at renderPage (C:\p\react-sticky-el\node_modules\next\dist\server\render.js:176:26)
    at Function.getInitialProps (C:\p\react-sticky-el\node_modules\next\dist\server\document.js:93:25)
    at _callee$ (C:\p\react-sticky-el\node_modules\next\dist\lib\utils.js:36:30)
    at tryCatch (C:\p\react-sticky-el\node_modules\regenerator-runtime\runtime.js:62:40)
    at Generator.invoke [as _invoke] (C:\p\react-sticky-el\node_modules\regenerator-runtime\runtime.js:296:22)
    at Generator.prototype.(anonymous function) [as next] (C:\p\react-sticky-el\node_modules\regenerator-runtime\runtime.js:114:21)
    at step (C:\p\react-sticky-el\node_modules\babel-runtime\helpers\asyncToGenerator.js:17:30)
    at C:\p\react-sticky-el\node_modules\babel-runtime\helpers\asyncToGenerator.js:35:14
    at new Promise (<anonymous>)
    at new F (C:\p\react-sticky-el\node_modules\core-js\library\modules\_export.js:35:28)
    at C:\p\react-sticky-el\node_modules\babel-runtime\helpers\asyncToGenerator.js:14:12
    at loadGetInitialProps (C:\p\react-sticky-el\node_modules\next\dist\lib\utils.js:70:17)

Any ideas? Many thanks in advance