In $MEAD_DIR/lib/Essence/IDF.pm, remove this code on lines 39-40:

unless (-e "$dbmname.dir") {
   die "$dbmname : No such dbm\n";
}

Immediately following this code on lines 43-45:

unless (dbmopen %nidf, $dbmname, 0666) {
   die "Cannot open DBM $dbmname\n";
}

insert this code:

unless (scalar(keys(%nidf))) {
   die "Empty DBM $dbmname";
}

So, the entire subroutine "open_nidf" should look like this:

sub open_nidf{
   my $dbmname = shift || $DEFAULT_DBMNAME;

   $dbmname = File::Spec->rel2abs($dbmname, $IDFDIR);

   if ($current_dbmname && $current_dbmname eq $dbmname) {
       return 1;
   }

   unless (dbmopen %nidf, $dbmname, 0666) {
       die "Cannot open DBM $dbmname";
   }

   unless (scalar(keys(%nidf))) {
       die "Empty DBM $dbmname";
   }

   $current_dbmname = $dbmname;

   return 1;
}