[O] org-koma-letter-template: Wrong type argument: char-or-string-p, nil

[Joseph Vidal-Rosset]

Hello,

I'm trying to export to koma class,  and especially to get the export to
koma letter but I meet always the same message: 

"org-koma-letter-template: Wrong type argument: char-or-string-p, nil"

and I do  not succeed to see  what is wrong in my  setup. Therefore your
help is welcome. 

Best wishes, 

Jo.