pg_basebackup: wal streaming can only be used in plain mode

In regards to pg_basebackup, I want to use "--format=tar" (for the purpose of sending the backup to stdout) but when I also try to use "--xlog-method=stream" to include some WAL files (without worrying about the wal_keep_segment parameter on the server) it tells me "wal streaming can only be used in plain mode".

I can't find any documentation regarding this conflict between "--format=tar" and "--xlog-method=stream". Can anyone provide any insight into why this combination is forbidden, or where it might be documented? Are there plans to support this combination in the future?

Thanks for any help