[exim] Negation not allowed with "acl"?

Hi,

Just compiled Exim 4.61 and received the following error on our existing
configuration:

2006-04-04 11:42:25 Exim configuration error in line 590 of /usr/local/exim/config: error in ACL: ACL error: negation is not allowed with "acl"

The relevant line of configuration is:

  warn ! acl = sender_is_customer
       message = X-Foreign-Sender: $sender_host_address

Is it no longer possible to use negation on acl statements such as above?
Maybe it was never supposed to be possible?

--
Dean Brooks
dean@???