[Exim] Expansion contains non-printing characters...

I had several errors in the log yesterday (all for the same message) for
users who had vacation messages set. The error is...

user@??? <user@???> D=uservacation
       T=uservacation: Expansion of "${if def:h_Subject: {Autoreply:
       $h_Subject:} {Automatic reply to your message}}" in
       uservacation transport contains non-printing characters

Here's the transport...

uservacation:
driver = autoreply
user = ${local_part}
once = ${home}/vacation
file = ${home}/vacation.txt
log = ${home}/vacation.log
once_repeat = 7d
to = "${sender_address}"
subject = "${if def:h_Subject: {Autoreply: $h_Subject:} {Automatic
reply to your message}}"

Folks who didn't have a vacation message set didn't have any problems
receiving the message, the folks who did have vacation messages set had the
transport successfully used for other messages. From that, I conclude that
there must have been something in the subject of the inbound message that
"caused the problem".

It does look like there might be a carriage return embedded in the subject.

How can I get exim to "not care" about the non-printing character in the 
macro expansion?    

Bruce

Bruce Bowler                             207.633.9600 (voice)
Research Associate                       207.633.9641 (fax)
Bigelow Laboratory for Ocean Sciences    bbowler@???
West Boothbay Harbor ME  04575           http://www.bigelow.org/