[exim] EXIM4 + greylistd on debian : whitelist problem

hello,
i am a newbie in exim an have a debian box with exim4 + greylistd set up.
i did everything as explained in howto and greylisting seems to work...
but whitelist file does not:/ i dont know what is wrong cause everything
seems to be left as with default cfg.
if i add following lines to acl/20_exim4(...)whitelist_local_deny
whitelisting seems to work:

  accept
    hosts = 64.233.182.0/24

but

    accept
    hosts =  ${if exists {/etc/greylistd/whitelist-hosts}\
                                 {/etc/greylistd/whitelist-hosts}{}}

doesnt seem to work...
(the file exists)

anyone has any suggestyions what may be the issue?