Bash completion and the $cur variable

Question

I have some programs for which I've created some bash completion scripts, and those seem to work OK. I'd like to add some tests, and though this approach looked promising, but I get unexpected failures. I instrument the completion script as

_cpthsv()
{
    local cur prev words cword split
    _init_completion -s || return

    # debugging
    echo
    echo "cur: '$cur'"
    echo "prev: '$prev'"
    echo "words: ${COMP_WORDS[*]}"
    :

and on the command-line

$ cpthsv --hel<tab>
cur: '--hel'
prev: 'cpthsv'
words: cpthsv --hel

which looks right (and completes to --help). But

$ COMP_WORDS=(cpthsv --hel) ; COMP_CWORD=1 ; _cpthsv ; echo ${COMPREPLY[*]}
cur: ''
prev: 'cpthsv'
words: cpthsv --hel

i.e., $cur variable is empty (and the completion fails because of this). I've looked at the _init_completion function which is supposed to set $cur but can't see anything obvious.

Would anyone know why $cur is set in the first scenario, but not the second; better, is there a workaround in the second case?