[bug?] word based string subscripting past the end

[Stephane Chazelas <stephane@xxxxxxxxxxxx>]

$ a='foo bar baz' zsh -c 'print -r - $a[(w)3,(w)3]'
baz
$ a='foo bar baz' zsh -c 'print -r - $a[(w)3,(w)4]'
baz

OK but:

$ a='foo bar baz' zsh -c 'print -r - $a[(w)4,(w)4]'
baz
$ a='foo bar baz' zsh -c 'print -r - $a[(w)4,(w)-1]'
baz

I'd have expected an empty output like for character
subscripting or array subscripting:

$ a='abc' zsh -c 'print -r - $a[4,4]'

$ a='abc' zsh -c 'print -r - $a[4,-1]'

$ zsh -c 'a=(a b c); print -r - $a[4,4]'

$ zsh -c 'a=(a b c); print -r - $a[4,-1]'

$

(with 5.9 and latest devel).

Cheers,
Stephane