{"action":"opened","issue":{"url":"https://api.github.com/repos/emberjs/ember.js/issues/13994","repository_url":"https://api.github.com/repos/emberjs/ember.js","labels_url":"https://api.github.com/repos/emberjs/ember.js/issues/13994/labels{/name}","comments_url":"https://api.github.com/repos/emberjs/ember.js/issues/13994/comments","events_url":"https://api.github.com/repos/emberjs/ember.js/issues/13994/events","html_url":"https://github.com/emberjs/ember.js/issues/13994","id":168790568,"number":13994,"title":"Error accessing block params > number of yielded items","user":{"login":"rwjblue","id":12637,"avatar_url":"https://avatars.githubusercontent.com/u/12637?v=3","gravatar_id":"","url":"https://api.github.com/users/rwjblue","html_url":"https://github.com/rwjblue","followers_url":"https://api.github.com/users/rwjblue/followers","following_url":"https://api.github.com/users/rwjblue/following{/other_user}","gists_url":"https://api.github.com/users/rwjblue/gists{/gist_id}","starred_url":"https://api.github.com/users/rwjblue/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/rwjblue/subscriptions","organizations_url":"https://api.github.com/users/rwjblue/orgs","repos_url":"https://api.github.com/users/rwjblue/repos","events_url":"https://api.github.com/users/rwjblue/events{/privacy}","received_events_url":"https://api.github.com/users/rwjblue/received_events","type":"User","site_admin":false},"labels":[{"url":"https://api.github.com/repos/emberjs/ember.js/labels/Bug","name":"Bug","color":"e10c02"},{"url":"https://api.github.com/repos/emberjs/ember.js/labels/Glimmer2","name":"Glimmer2","color":"1d76db"}],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":0,"created_at":"2016-08-02T03:16:51Z","updated_at":"2016-08-02T03:16:51Z","closed_at":null,"body":"Given the following templates:\r\n\r\n```js\r\n{{#foo-bar as |hola index|}}\r\n  {{hola}} - {{index}}\r\n{{/foo-bar}}\r\n```\r\n\r\n```hbs\r\n{{! app/templates/components/foo-bar.hbs }}\r\n\r\n{{yield \"hi\"}}\r\n```\r\n\r\nThe following error is triggered:\r\n\r\n```\r\nTypeError: Cannot read property 'tag' of undefined\r\n    at new MapperReference (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:40994:33)\r\n    at Object.map (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:41010:16)\r\n    at OptimizedCautiousAppendOpcode.normalize (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:44144:38)\r\n    at OptimizedCautiousAppendOpcode.evaluate (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:43807:35)\r\n    at VM.execute (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:49871:28)\r\n    at Template.render (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:49397:23)\r\n    at render (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:10152:46)\r\n    at runInTransaction (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:20178:9)\r\n    at transaction (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:10174:27)\r\n    at Renderer._transaction (https://s3.amazonaws.com/builds.emberjs.com/alpha/ember.debug.js:10181:11)\r\n```\r\n\r\n[Demo here](https://jsbin.com/mosise/edit?html,js,output)"}}
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