I get the error message in the title while performing a COPY ... FROM STDIN query.

I've found the error message in copy.go, and indeed, message type 'S' is not handled there.
If I read postgresql's source (doc/src/sgml/protocol.sgml) right, message type 'S' is a ParameterStatus message, and their public documentation ( https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-COPY ) does say that "it is possible for NoticeResponse and ParameterStatus messages to be interspersed between CopyData messages; frontends must handle these cases".

I haven't been able to determine what in the table structure triggers this kind of message.