Replacing in function mml-parse-1 (mml.el, line 284):

(setq tag (list 'part '(type . "text/plain"))
       no-markup-p t
       warn t)

with

(setq tag `(part (type . ,"text/plain"))
      no-markup-p t
      warn t)

seems to fix this bug. It's not clear to me why though.