Why I Cannot Generate Oauth Tokens via API For Social or SAML Authenticated Users in Ansible Tower ?

Issue

When generating a Oauth Token for the Social or SAML Authenticated user by specifying the user's username and password in the API call, I'm getting "Authenticated credentials were not provided" error.

curl -ku USERNAME:PASSWORD -H "Content-Type: application/json" -X POST -d '{"description":"Tower CLI", "application":null, "scope":"write"}' https://tower.example.com/api/v2/users/18/personal_tokens/ |python -m json.tool

{
    "detail": "Authentication credentials were not provided. To establish a login session, visit /api/login/."
}