Template render error: (unknown path) [Line 8, Column 25]

when I start `hexo s -g`, I got this error
```
Error: Unable to call `the return value of (posts["first"])["updated"]["toISOString"]`, which is undefined or falsey
at Object._prettifyError (/Users/kobetian/blog/node_modules/nunjucks/src/lib.js:36:11)
at /Users/kobetian/blog/node_modules/nunjucks/src/environment.js:561:19
at Template.root [as rootRenderFunc] (eval at _compile (/Users/kobetian/blog/node_modules/nunjucks/src/environment.js:631:18), :214:3)
at Template.render (/Users/kobetian/blog/node_modules/nunjucks/src/environment.js:550:10)
at Hexo.module.exports (/Users/kobetian/blog/node_modules/hexo-generator-feed/lib/generator.js:40:22)
at Hexo.tryCatcher (/Users/kobetian/blog/node_modules/bluebird/js/release/util.js:16:23)
at Hexo. (/Users/kobetian/blog/node_modules/bluebird/js/release/method.js:15:34)
at /Users/kobetian/blog/node_modules/hexo/lib/hexo/index.js:318:20
at tryCatcher (/Users/kobetian/blog/node_modules/bluebird/js/release/util.js:16:23)
at MappingPromiseArray._promiseFulfilled (/Users/kobetian/blog/node_modules/bluebird/js/release/map.js:68:38)
at MappingPromiseArray.PromiseArray._iterate (/Users/kobetian/blog/node_modules/bluebird/js/release/promise_array.js:115:31)
at MappingPromiseArray.init (/Users/kobetian/blog/node_modules/bluebird/js/release/promise_array.js:79:10)
at MappingPromiseArray._asyncInit (/Users/kobetian/blog/node_modules/bluebird/js/release/map.js:37:10)
at _drainQueueStep (/Users/kobetian/blog/node_modules/bluebird/js/release/async.js:97:12)
at _drainQueue (/Users/kobetian/blog/node_modules/bluebird/js/release/async.js:86:9)
at Async._drainQueues (/Users/kobetian/blog/node_modules/bluebird/js/release/async.js:102:5)
at Immediate.Async.drainQueues [as _onImmediate] (/Users/kobetian/blog/node_modules/bluebird/js/release/async.js:15:14)
at processImmediate (internal/timers.js:439:21)
```
Anyone can help me, Thx a lot!

Reproduce the failure with `hexo s -g`, then inspect `hexo-generator-feed/lib/generator.js` and the Nunjucks template location reported as line 8, column 25. Check how `posts["first"]["updated"]` is populated before `toISOString` is called; done means the documentation site generates successfully without this error.

Written by the indexing model from the issue text.