[Zope] Sendmail: Problem with string.join

OK, what's wrong with this then? <dtml-let email_list=dbGetRumourInterest> <dtml-let followup_list=dbGetFollowups> <dtml-if email_list> <dtml-sendmail mailhost="MailHost"> To: <dtml-var expr="_.string.join(email_list,',')"> From: ozrail@bigpond.net.au etc... I found the string.join example in an email from Dieter, but when I try this I get an error: first argument must be a sequence of strings. When I run the SQL standalone, it returns what looks like a sequence of strings to me. Any clues out there? Steve