failing to understand/use lambda

I want to create a class which I can provide a "reporter" callable
so it can notify me of what it is doing. Unfortunately, I can't get
it to work and don't understand what I'm doing wrong.
Below is a cut-down example to illustrate.

package require lambda 1

oo::class create Store {
    variable Filename
    variable Reporter
}

oo::define Store constructor {filename {reporter ""}} {
    set Filename $filename
    if {$reporter eq ""} {
        set Reporter [lambda {message} {}]
    } else {
        set Reporter $reporter
    }
}

oo::define Store method add {args} {
    puts "action: adding [llength $args] new files"
    $Reporter "adding [llength $args] new files"
}

set str [Store new test.dat [lambda {message} { puts "reporter: $message" }]]

$str add one.txt two.txt three.txt

Expected output:

action: adding 3 new files
reporter: adding 3 new files

Actual output:

action: adding 3 new files
invalid command name "::apply {message { puts "reporter: $message" }}"
    while executing
"$Reporter "adding [llength $args] new files""
    (class "::Store" method "add" line 3)
    invoked from within
"$str add one.txt two.txt three.txt"