I am trying to use ox-koma-letter for my correspondence. I have a
functioning test file that produces a nice pdf. But I would like to
set it up so as to have one letter per headline, and do subtree export
to produce the pdf. 

I am attaching the test file for the purpose. When I export it using
C-c @ C-c C-e k o, this file gives me the following error:

let*: Wrong type argument: char-or-string-p, nil

Have I made a mistake or is there a bug somewhere?

Vikas