Given JavaDoc:

    public AclEntry.Builder setFlags(Set<AclEntryFlag> flags)

    Sets the flags component of this builder. On return, the flags
    component of this builder is a copy of the given set.

    Returns:
    this builder

    Throws:

    ClassCastException - if the set contains elements that are not of type
                         AclEntryFlag

I would expect an empty set argument in "flags" to work (and so does the JDK "sample"
app AclEdit.java, by the way), but no:

> java -cp . AclEdit A2=Alle:write_data/append_data:deny derby.log

    Exception in thread "main" java.lang.IllegalArgumentException: Collection is empty
     at java.util.EnumSet.copyOf(EnumSet.java:166)
           at java.nio.file.attribute.AclEntry$Builder.setFlags(AclEntry.java:216)
         at AclEdit.parseAceString(AclEdit.java:162)
     at AclEdit.main(AclEdit.java:294)

Alan Bateman confirmed this is a code bug.