Why are some authors assigned to "NULL AUTHOR_ID" (A9999999999)?

A9999999999 is the NULL author — a single catch-all ID for authorships that didn’t go through OpenAlex’s disambiguation process. You’ll most often see it in the data snapshot.

An authorship lands here when:

• no author name was received from the source,
• the name was too short or too long to disambiguate reliably,
• the name matched an ignored phrase (like “Unknown Author”), or
• the author asked to have their disambiguated profile removed (all their works are reassigned to NULL, removing the profile).

There’s a second special ID, A5317838346, for deleted authors — used when an author ID is removed because it no longer has any works.

For the full explanation of how OpenAlex assigns author IDs, see Author disambiguation in the docs.