"Dist currently only works with Git or Mercurial repos" when running simple example

I'm trying to setup a Python project which will compile a meson subproject and provide a Python interface to it.

Following the example on the website, I created a meson.build which simply declares a subproject, and the git repository for that project has been cloned into ./subprojects/<dep_name>, relative to my project's root:

project('myprojectname')

dep_name_proj = subproject('dep_name')

When I run python -m build, I get the error:

Dist currently only works with Git or Mercurial repos
ERROR Backend subprocess exited when trying to invoke build_sdist

Does this mean to imply that I cannot use meson-python unless /my/ project is in a Git or Mercurial repository? If not, what am I doing wrong?