The "path" argument must be of type string. Received type undefined

<h1>The &#34;path&#34; argument must be of type string. Received type undefined</h1>
<pre>TypeError [ERR_INVALID_ARG_TYPE]: The &#34;path&#34; argument must be of type string. Received type undefined
    at validateString (internal/validators.js:125:11)
    at Object.join (path.js:1147:7)
    at /root/Dropbox/routes/default.js:22:58
    at Layer.handle [as handle_request] (/root/Dropbox/node_modules/express/lib/router/layer.js:95:5)
    at next (/root/Dropbox/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/root/Dropbox/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/root/Dropbox/node_modules/express/lib/router/layer.js:95:5)
    at /root/Dropbox/node_modules/express/lib/router/index.js:281:22
    at param (/root/Dropbox/node_modules/express/lib/router/index.js:354:14)
    at param (/root/Dropbox/node_modules/express/lib/router/index.js:365:14)</pre>