`us-east1-a` doesn't exist

[yujikiriki]

Using this yaml from the Anthos tutorial, changing the zone from us-central1 to us-east1 generated this error when executing terraform apply:

Error: Error loading zone 'us-east1-a': googleapi: Error 404: The resource 'projects/[project-id]/zones/us-east1-a' was not found
, notFound

Reviewing the terraform plan I found that the zone was automatically generated.

It looks like the us-east1-a zone doesn't exist, only the zones b, c, d.

Is this an issue or am I missing something?