[Exim] exim 4.10 and libwrap

I'm looking at section 4.6 use of tcpwrappers. Specifically the
option to put entries into /etc/hosts.allow

It says "Consult the tcpwrappers documentation for further details."

Our version of tcpwrappers includes the "twist" option.

But if I attempt an entry in /etc/hosts.allow such as:

exim : my.test.address : twist /bin/echo 421 foo bar

(this is the specific example in the libwrap documentation), or
anything similar, then when I attempt to connect from my.test.address
the following gets logged in /var/log/messages :

... exim: twist my.test.address to /bin/echo 421 foo bar
... exim: warning: /etc/hosts.allow, line 21: twist_option:
dup: Bad file descriptor

No big deal, but I was wondering whether this ought to work (it works
as advertised for e.g sshd) or not? Of course I wanted to do
something more interesting than /bin/echo, but until I can get that to
work there can be no progress...

cheers