Evolution can't sign emails with S/MIME signature.

This bug was filed and rejected as NOTXIMIAN at Ximian's bugzilla:

http://bugzilla.ximian.com/show_bug.cgi?id=69205

In Evolution 2.2.0 and the latest Hoary release, the correct Thawte signature is
imported and assigned to the mail account. Upon trying to send an S/MIME signed
message, the following error appears:

Could not create message.

Because "Cannot add SMIMEEncKeyPrefs attribute", you may need to select
different mail options.

This message does not appear if only encrypt is selected, but appears every time
that sign is selected. Also, the same signature was previously used without
troubles on Evo 2.0.4 under Mandrake 10.2b3, so it's not a problem with the
signature.

FWIW, the poster of the Ximian bug was running Debian as well. I wonder if this
is a problem with Debian's libnss packages.