problems with index_format

[David Bear <David.Bear@xxxxxxx>]

I'm trying the following folder hook

####
folder-hook mutt-sent set sort=date
folder-hook mutt-sent set index_format="%C %d %t %s"
######


However, when I open the mutt-sent folder mutt gives me that message 

%d unknown variable

Of course the display only shows the current message number.

This seems more difficult than it should be. Advice please?
-- 
David Bear
phone:  480-965-8257
fax:    480-965-9189
College of Public Programs/ASU
Wilson Hall 232
Tempe, AZ 85287-0803
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