wal-e backup-push not terminating (waiting for required WAL segments to be archived)

Question

Trying to setup wal-e for postgres.

Following various tutorials and I'm at a point to finally do a first backup of a clean 9.6 postgres install.

Followed some tutorials and finally read to do an initial wal-e backup-push, as follows:

sudo -u postgres -i envdir /etc/wal-e.d/env wal-e backup-push /var/lib/postgresql/9.6/main

I'd expect the command to terminate rather quickly, since it's an empty database.

However it seems to wait indefinitely. Showing waiting for required WAL segments to be archived

 postgres@postgres:~$ envdir /etc/wal-e.d/env wal-e backup-push /var/lib/postgresql/9.6/main
wal_e.main   INFO     MSG: starting WAL-E
        DETAIL: The subcommand is "backup-push".
        STRUCTURED: time=2017-05-26T11:45:52.138889-00 pid=10426
wal_e.operator.backup INFO     MSG: start upload postgres version metadata
        DETAIL: Uploading to s3://xxxxxx/basebackups_005/base_000000010000000000000060_00000040/extended_version.txt.
        STRUCTURED: time=2017-05-26T11:45:52.719220-00 pid=10426
wal_e.operator.backup INFO     MSG: postgres version metadata upload complete
        STRUCTURED: time=2017-05-26T11:45:52.929696-00 pid=10426
wal_e.worker.upload INFO     MSG: beginning volume compression
        DETAIL: Building volume 0.
        STRUCTURED: time=2017-05-26T11:45:53.075771-00 pid=10426
wal_e.worker.upload INFO     MSG: begin uploading a base backup volume
        DETAIL: Uploading to "s3://xxxxxx/basebackups_005/base_000000010000000000000060_00000040/tar_partitions/part_00000000.tar.lzo".
        STRUCTURED: time=2017-05-26T11:45:53.752390-00 pid=10426
wal_e.worker.upload INFO     MSG: finish uploading a base backup volume
        DETAIL: Uploading to "s3://xxxxxx/basebackups_005/base_000000010000000000000060_00000040/tar_partitions/part_00000000.tar.lzo" complete at 9106.47KiB/s. 
        STRUCTURED: time=2017-05-26T11:45:54.327037-00 pid=10426
NOTICE:  pg_stop_backup cleanup done, waiting for required WAL segments to be archived
WARNING:  pg_stop_backup still waiting for all required WAL segments to be archived (60 seconds elapsed)
HINT:  Check that your archive_command is executing properly.  pg_stop_backup can be canceled safely, but the database backup will not be usable without all the WAL segments.
WARNING:  pg_stop_backup still waiting for all required WAL segments to be archived (120 seconds elapsed)
HINT:  Check that your archive_command is executing properly.  pg_stop_backup can be canceled safely, but the database backup will not be usable without all the WAL segments.

To be honest I'm a bit out of my depth here. Afaik, the above should do an initial backup, but not wait for WAL-files which are obviously continuously updated. (I've got archive_mode=on in my postgres config and set archive_command = 'envdir /etc/wal-e.d/env wal-e wal-push %p' which should do the incremental pushes. Again afaik.).

How can I get this initial backup command to finish?