mutable `*$ftd.empty` doesn't work

[amitu]

With the following code there is no output for last statement:

-- string $username: *$ftd.empty

-- ftd.text-input:
placeholder: enter username
$on-input$: $ftd.set-string($a = $username, v = $VALUE)

-- ftd.text: $username

We expect that the username will echo in the ftd.text field. It works if we change the first line to: -- string $username: hello.