Comment preview doesn't work when a sheet is applied to a document

Previewing a comment before adding it to a document that has a sheet applied renders the document instead of the comment. In order to reproduce this, the sheet must be applied using the new sheet system (so without using the include macro) and it musn't specify an action, i.e. the sheet must be compatible with the preview action. The quickest way to check this problem is to preview a comment on a blog post.