Postgred pg_notify/listen only working if channel name is lower case

I've read https://www.postgresql.org/docs/9.6/static/sql-notify.html and the channel name is just described as an 'identifier'.

I'm using NodeJS and pg https://www.npmjs.com/package/pg to access postgres.

If I use a lower case word in both, e.g. pg_notify('foo', ... and LISTEN foo it works. I've tested all combinations:

pg_notify  LISTEN  outcome
lower      lower   works
lower      upper   works
upper      lower   fails
upper      upper   fails

Is this a bug, or is it a logical result of being an 'identifier'? (If so, should this be documented on pg_notify page?)

via chrisdew