[exim] Closed mailing lists in exim-4.94.2

We have been using a router as suggested by the manual for
closed lists restricting posts to some senders but now they
are rejected with
Tainted filename for search

 lists_post:
  driver = redirect
  domains = lists.example
  senders = ${if exists {/usr/lists/$local_part}\
             {lsearch;/usr/lists/$local_part}{*}}
  errors_to = ${quote_local_part:$local_part-request}@???
  no_more

But I cant get the senders to be detainted using a lookup
in the file of allowed senders. An example will be appriciated.

Something like
${lookup{$sender_data}lsearch{/usr/lists/$local_part}}

hmk