Although Mule is based on GNU Emacs, they are slightly different at the level of emacs lisp. Mule is quite different from Nemacs.
Enami TSUGUTOMO <firstname.lastname@example.org> points out the incompatibility between Mule and Nemacs/Emacs as follows:
definitions (number of arguments, etc.)
of the internal representation between Mule and Nemacs. Their byte code is compatible, however. (isn't it? :-)
Emacs they are based on. (only current-column, maybe.)