Headings with codespans cause error output

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Given a glossary with a term

glossary.md

# Glossary

## Term
...a term

and a term-occurrence in a section with a headline containing a code-span

document.md

## `FooClass`

Term

Then when generating an index file with option generateFiles.indexFile the term produces an untitled index link reference

book-index.md

## Term 
[][9] 

[9]: ./glossary.md#term ... a term

which causes an error output by remark-stringify.