Can mail an article but not a thread

I am using tin version 1.62 and have just noticed that when I mail a
thread as opposed to an article to someone (usually me!) it doesn't
work.

I have tin set up to use mutt interactively as its mailer.  When I
mail an article it all works as expected, mutt pops up, I can edit the
article and send it.

However when I mail a thread what happens is that I'm asked to enter
or confirm the address and then I get the message "leaving external
mailer" and I'm returned to tin.

I have the following line in my tinrc file:-

    default_mailer_format=mutt -s "%S" -i "%F" "%T"

-- 
Chris Green ([email protected])

    "Never ascribe to malice that which can be explained by incompetence."