hexo server的时候报Template render error : (unknown path) [Line 7, Column 23]

Template render error: (unknown path) [Line 7, Column 23]
Error: Unable to call the return value of (posts["first"])["updated"]["toISOString"], which is undefined or falsey
at Object.exports.prettifyError (D:\itxuye\node_modules\nunjucks\src\lib.js:34:15)
at D:\itxuye\node_modules\nunjucks\src\environment.js:485:31
at root [as rootRenderFunc](eval at %28D:itxuyenode_modulesnunjuckssrcenvironment.js:564:24%29, :161:3)
at Obj.extend.render (D:\itxuye\node_modules\nunjucks\src\environment.js:478:15)
at Hexo.module.exports (D:\itxuye\node_modules\hexo-generator-feed\lib\generator.js:28:22)
at Hexo.tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at Hexo. (D:\itxuye\node_modules\bluebird\js\release\method.js:15:34)
at D:\itxuye\node_modules\hexo\lib\hexo\index.js:337:24
at tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at MappingPromiseArray._promiseFulfilled (D:\itxuye\node_modules\bluebird\js\release\map.js:57:38)
at MappingPromiseArray.PromiseArray._iterate (D:\itxuye\node_modules\bluebird\js\release\promise_array.js:113:31)
at MappingPromiseArray.init (D:\itxuye\node_modules\bluebird\js\release\promise_array.js:77:10)
at new MappingPromiseArray (D:\itxuye\node_modules\bluebird\js\release\map.js:25:10)
at map (D:\itxuye\node_modules\bluebird\js\release\map.js:152:12)
at Function.Promise.map (D:\itxuye\node_modules\bluebird\js\release\map.js:160:12)
at D:\itxuye\node_modules\hexo\lib\hexo\index.js:334:20
at tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at Promise._settlePromiseFromHandler (D:\itxuye\node_modules\bluebird\js\release\promise.js:504:31)
at Promise._settlePromise (D:\itxuye\node_modules\bluebird\js\release\promise.js:561:18)
at Promise._settlePromise0 (D:\itxuye\node_modules\bluebird\js\release\promise.js:606:10)
at Promise._settlePromises (D:\itxuye\node_modules\bluebird\js\release\promise.js:685:18)
at Async._drainQueue (D:\itxuye\node_modules\bluebird\js\release\async.js:138:16)
02:09:00.737 DEBUG Generator: asset
02:09:00.741 DEBUG Database saved
02:09:00.742 FATAL (unknown path) [Line 7, Column 23]
Error: Unable to call the return value of (posts["first"])["updated"]["toISOString"], which is undefined or falsey
Template render error: (unknown path) [Line 7, Column 23]
Error: Unable to call the return value of (posts["first"])["updated"]["toISOString"], which is undefined or falsey
at Object.exports.prettifyError (D:\itxuye\node_modules\nunjucks\src\lib.js:34:15)
at D:\itxuye\node_modules\nunjucks\src\environment.js:485:31
at root [as rootRenderFunc](eval at %28D:itxuyenode_modulesnunjuckssrcenvironment.js:564:24%29, :161:3)
at Obj.extend.render (D:\itxuye\node_modules\nunjucks\src\environment.js:478:15)
at Hexo.module.exports (D:\itxuye\node_modules\hexo-generator-feed\lib\generator.js:28:22)
at Hexo.tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at Hexo. (D:\itxuye\node_modules\bluebird\js\release\method.js:15:34)
at D:\itxuye\node_modules\hexo\lib\hexo\index.js:337:24
at tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at MappingPromiseArray._promiseFulfilled (D:\itxuye\node_modules\bluebird\js\release\map.js:57:38)
at MappingPromiseArray.PromiseArray._iterate (D:\itxuye\node_modules\bluebird\js\release\promise_array.js:113:31)
at MappingPromiseArray.init (D:\itxuye\node_modules\bluebird\js\release\promise_array.js:77:10)
at new MappingPromiseArray (D:\itxuye\node_modules\bluebird\js\release\map.js:25:10)
at map (D:\itxuye\node_modules\bluebird\js\release\map.js:152:12)
at Function.Promise.map (D:\itxuye\node_modules\bluebird\js\release\map.js:160:12)
at D:\itxuye\node_modules\hexo\lib\hexo\index.js:334:20
at tryCatcher (D:\itxuye\node_modules\bluebird\js\release\util.js:16:23)
at Promise._settlePromiseFromHandler (D:\itxuye\node_modules\bluebird\js\release\promise.js:504:31)
at Promise._settlePromise (D:\itxuye\node_modules\bluebird\js\release\promise.js:561:18)
at Promise._settlePromise0 (D:\itxuye\node_modules\bluebird\js\release\promise.js:606:10)
at Promise._settlePromises (D:\itxuye\node_modules\bluebird\js\release\promise.js:685:18)
at Async._drainQueue (D:\itxuye\node_modules\bluebird\js\release\async.js:138:16)

What is the problem?How can i solve it ~~~

Start with hexo-generator-feed/lib/generator.js at the reported line and inspect how posts["first"].updated is obtained before the Nunjucks template call. Reproduce with hexo server and determine what input produces the missing updated value; done means generation completes without the Template render error.