[EXIM] Expansion problem with substr in 1.92

Hello,
I am having difficulties with the setup as explained in the Exim specification
on my FreeBSD host. The Exim version is 1.92, my host is running
FreeBSD 2.2.6-STABLE.

I use the following in the Exim configuration file for my IMAP folders, as
explained in the specification:

local_imapfolder_delivery:
  driver = pipe;
  command = "/usr/local/cyrus/bin/deliver \
            -m ${substr_1:${$local_part_suffix}} -- ${local_part}"
  user = cyrus
  group = mail
  return_output
  log_output
  prefix =
  suffix =

All deliveries without folders are ok, since they use another transport, namely
local_imap_delivery.

Whenever I want to use the folder delivery (for example jh.test@???) I get
the following error:
1998-05-01 23:45:04 0yVPPs-0000on-00 == jh.test@??? T=local_imapfolder_delivery defer (0): Expansion of "${substr_1:${$local_part_suffix}}" from command "/usr/local/cyrus/bin/deliver -m ${substr_1:${$local_part_suffix}} -- ${local_part}" in local_imapfolder_delivery transport failed: letter or digit expected after ${

What is wrong??
Thanks for any help, --jeroen

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