Major Description
RFC 3659, section 7.9 states "Facts requested that are not supported, or that are inappropriate to the file or directory being listed should simply be omitted from the MLSx output. This is not an error. Note that where no factname arguments are present, the client is requesting that only the file names be returned."
org.apache.ftpserver.command.impl.OPTS_MLST.validateSelectedTypes() specifically returns null if one of the types requested is not found, and a null return generates the 501 from execute().