[exim] Expansion contains non-printing character

Hi,

I have an autoreply transport that gets its data from MySQL:

subject = ${expand:${lookup mysql {MYSQL_ARSUBJECT}{$value}{Re:
$header_subject:}}}

The result of the database lookup is expanded further, because it may
contain expansion variables like "$header_subject" itself.

Now when the autoreply is triggered by a message that has a subject like
this:

Subject: =?iso-8859-1?Q?RE:_Cursus_intu=EFtieve_ontwikkeling?=

delivery fails with the following error:

Expansion of "${expand:${lookup mysql {SELECT arsubject FROM virtual
WHERE username='${quote_mysql:$local_part}' AND
domain='${quote_mysql:$domain}'}{$value}{Re: $header_subject:}}}" in
mysql_reply transport contains non-printing character 195

So, I guess I should somehow quote the final result of the expansion(s)
before I can use it as a subject.

My question is: which quoting operator would be most suitable for this?
Would just plain ${quote:<string>} do? Any other things I should be
aware of?

Best regards,

Martijn Grendelman