Question about language specific mappings

More precisely question is about unmapping. I am using keymap to edit
texts in Russian. In my vimrc I have

set keymap=russian-jcukenwin
set iminsert=0
set imsearch=-1

When langmap mappings are enabled it translates `#' to `№' as expected.
It is not very useful in markdown files. So I put

lunmap <buffer> #

in ~/.vim/after/ftplugin/markdown.vim. Then strange things begin to
happen. When I open markdown document from running vim it shouts

E31: No such mapping

and points on that line. But after that mapping exists. When I start
to edit markdown from command line everything is fine.

Using autocommand instead of script in "after" directory leads to the
same.

The exact question is how to properly unmap this kind of mapping?
Thanks for any help.

--
Dmitri Vereshchagin

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