HTML Export fails when there is non-existing attachment

HTML Export fails when there is non-existing attachment. I confirmed it happens both in the latest Confluence Server and in the latest Confluence Cloud.

How I reproduced:

1. Install vanilla Confluence Server 5.7
2. Create a Space
3. Create a Page in the Space
4. Attache a file on the Page
5. Back Up Confluence Data after unchecking "Back up attachments"
6. Restore Confluence Data from the backup file, so that the page has an attachment which does not exist actually
7. Go to the Space Tool and do HTML Export, which will end in failure

Workaround:

Delete such non-existing attachments as described in the following page before doing HTML Export:
https://confluence.atlassian.com/display/CONF52/Deleting+an+Attachment