When exporting markdown with ToC, if heading numbering is not enabled, it
would fail with the attached backtrace. The cause is that the function
`org-md--build-toc' in `ox-md.el' does not properly handle this case which
effectively results in a function call `(format "%d." nil)'. In my patch,
when exporting markdown with ToC, if heading numbering is not enabled, ToC
will be shown as an unordered list.