misunderstaning of switch command

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

    const UNCOMPRESSED U
    const ZLIB_COMPRESSED Z
    const SAME_AS_PREV =
    set filename somefile.txt
    set action "added"
    set kind Z
    switch $kind {
        $::SAME_AS_PREV { puts "unchanged \"$filename\"" }
        $::UNCOMPRESSED { puts "$action \"$filename\"" }
        $::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
        default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
    }

What am I doing wrong?