It is not an error and it falls back to IPv4.
You can explicitly chose protocol by --proto
option
Where
option can be: udp, udp4 or udp6 .. or tcp, tcp4 or tcp6
The manual has this:
The Manual wroteproto
indicates the protocol to use when connecting with the
remote, and may be "tcp" or "udp".
For forcing IPv4 or IPv6 connection suffix tcp or udp
with 4/6 like udp4/udp6/tcp4/tcp6.
