mbsync, SASL and Gmail

Slightly more esoteric than usual; I’m posting this for my benefit as much as anyone else’s!

I was just trying to set up mbsync to work with Gmail. I’d got most of the config copied from another provider, but kept getting an error about an invalid parameter:

Error performing SASL authentication step: SASL(-7): invalid parameter supplied: Parameter Error in /System/Volumes/Data/SWE/macOS/BuildRoots/533514bb11/Library/Caches/com.apple.xbs/Sources/passwordserver_saslplugins/passwordserver_saslplugins-194/plain_clienttoken.c near line 195