Special characters in subsection names break links in table of contents

[olliemath]

Adding a subsection with non alphanumeric characters, like ### Don't panic! generates html like this:

<h3 id="don-t-panic-">Don't panic!</h3>

but the corresponding entry in the TOC for that page is missing the hyphens:

<a href="this_page_address/#dont-panic">Don't panic!</a>

This results in a TOC with sporadic dead links.