执行 hexo generate时报错，Template render error: (unknown path) [Line 8, Column 25]

[cxwen]

$ hexo clean && hexo generate
INFO  Start processing
FATAL Something's wrong. Maybe you can find the solution here: https://hexo.io/docs/troubleshooting.html
Template render error: (unknown path) [Line 8, Column 25]
  Error: Unable to call `the return value of (posts["first"])["updated"]["toISOString"]`, which is undefined or falsey
    at Object._prettifyError (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/nunjucks/src/lib.js:36:11)
    at /Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/nunjucks/src/environment.js:561:19
    at Template.root [as rootRenderFunc] (eval at _compile (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/nunjucks/src/environment.js:631:18), <anonymous>:214:3)
    at Template.render (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/nunjucks/src/environment.js:550:10)
    at Hexo.module.exports (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/hexo-generator-feed/lib/generator.js:40:22)
    at Hexo.tryCatcher (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/util.js:16:23)
    at Hexo.<anonymous> (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/method.js:15:34)
    at /Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/hexo/lib/hexo/index.js:318:20
    at tryCatcher (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/util.js:16:23)
    at MappingPromiseArray._promiseFulfilled (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/map.js:68:38)
    at MappingPromiseArray.PromiseArray._iterate (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/promise_array.js:115:31)
    at MappingPromiseArray.init (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/promise_array.js:79:10)
    at MappingPromiseArray._asyncInit (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/map.js:37:10)
    at _drainQueueStep (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/async.js:97:12)
    at _drainQueue (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/async.js:86:9)
    at Async._drainQueues (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/async.js:102:5)
    at Immediate.Async.drainQueues [as _onImmediate] (/Users/chengxiaowen/go/src/github.com/cxwen/cxwen-blog/node_modules/bluebird/js/release/async.js:15:14)
    at processImmediate (internal/timers.js:439:21)