List/members/create.json returns error code 89




I am trying to add a member to a list (that the user owns and was created using the API via a POST request to lists/create.json), by making a POST request to lists/members/create.json.

I send the slug, owner_screen_name and screen_name, and always get error code 89 back like so “errors”:[{“code”:89,“message”:“Invalid or expired token.”}].

I have tried getting new access tokens, and still see this issue. what am i missing?