lookup error at router

I my users api  - > https://github.com/shubham1507/school/blob/trial/users/urls.py

HTTP 200 OK  Allow: GET, HEAD, OPTIONS  Content-Type: application/json  Vary: Accept    {      "teacher": "http://127.0.0.1:8000/users/teacher/",      "student": "http://127.0.0.1:8000/users/student/"  }



whenever I hit to either of the link 
"http://127.0.0.1:8000/users/teacher/",or 
 "http://127.0.0.1:8000/users/student/" then 


Could not resolve URL for hyperlinked relationship using view name "user-detail". You may have failed to include the related model in your API, or incorrectly configured the `lookup_field` attribute on this field.

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