Cannot read property 'belongsToMany' of undefined

db.Hashtag 가 왜 undefined로 나올까요 ㅠㅜ

에러메세지 입니다.

TypeError: Cannot read property 'belongsToMany' of undefined

    at Function.Hashtag.associate (D:\005_Study\React\react-nodebird\prepare\back\models\hashtag.js:12:16)

    at D:\005_Study\React\react-nodebird\prepare\back\models\index.js:19:19

    at Array.forEach (<anonymous>)

    at Object.<anonymous> (D:\005_Study\React\react-nodebird\prepare\back\models\index.js:17:17)

    at Module._compile (internal/modules/cjs/loader.js:1138:30)

    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)

    at Module.load (internal/modules/cjs/loader.js:986:32)

    at Function.Module._load (internal/modules/cjs/loader.js:879:14)

    at Module.require (internal/modules/cjs/loader.js:1026:19)

    at require (internal/modules/cjs/helpers.js:72:18)

hashtag.js입니다.