rsync giving "Directory has vanished" when there are multiple (existing) source directories

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http://unix.stackexchange.com – This is happening to me: $ rsync -av ./foo ./bar $DEST sending incremental file list directory has vanished: "/path/to/foo" foo exists and is regular a regular dir, and rsync -av ./foo $DEST works, same for bar. Is "directory has vanished" just how rsync tells me that it doesn't know how to work with multiple sources? Or am I importantly misinterpretting the (long) man page? (HowTos)