https://en.wikipedia.org/wiki/List_of_Internet_Relay_Chat_commands#USER

When attempting to connect to freenode, if the realname configuration is set to an empty string, freenode responds that USER does not have enough parameters. This is regardless of whether you are registered or SASL authentication succeeded.

I couldn't find whether this is invalid in the specification; it only calls out that names can contain spaces.

https://tools.ietf.org/html/rfc2812#section-3.1.3