Error: No default engine was specified and no extension was provided.

   at new View (/Users/Adam/Sites/Imooc/node_modules/express/lib/view.js:41:42)

   at Function.app.render (/Users/Adam/Sites/Imooc/node_modules/express/lib/application.js:499:12)

   at ServerResponse.res.render (/Users/Adam/Sites/Imooc/node_modules/express/lib/response.js:900:7)

   at app.get.res.render.title (/Users/Adam/Sites/Imooc/app.js:12:6)

   at Layer.handle [as handle_request] (/Users/Adam/Sites/Imooc/node_modules/express/lib/router/layer.js:76:5)

   at next (/Users/Adam/Sites/Imooc/node_modules/express/lib/router/route.js:100:13)

   at Route.dispatch (/Users/Adam/Sites/Imooc/node_modules/express/lib/router/route.js:81:3)

   at Layer.handle [as handle_request] (/Users/Adam/Sites/Imooc/node_modules/express/lib/router/layer.js:76:5)

   at /Users/Adam/Sites/Imooc/node_modules/express/lib/router/index.js:227:24

   at Function.proto.process_params (/Users/Adam/Sites/Imooc/node_modules/express/lib/router/index.js:305:12) 

请问这是什么原因