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

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type undefined
    at assertPath (path.js:39:11)
    at Object.join (path.js:1155:7)
    at /home/ubuntu/Dropbox/routes/default.js:22:58
    at Layer.handle [as handle_request] (/home/ubuntu/Dropbox/node_modules/express/lib/router/layer.js:95:5)
    at next (/home/ubuntu/Dropbox/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/home/ubuntu/Dropbox/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/home/ubuntu/Dropbox/node_modules/express/lib/router/layer.js:95:5)
    at /home/ubuntu/Dropbox/node_modules/express/lib/router/index.js:281:22
    at param (/home/ubuntu/Dropbox/node_modules/express/lib/router/index.js:354:14)
    at param (/home/ubuntu/Dropbox/node_modules/express/lib/router/index.js:365:14)