DNS Filter

Hi just wondering how the following in the Fortigate Cookbook actually means.

You configured DNS Filtering, but it is not working

"Verify that DNS Filter is enabled in a policy and SSL Inspection has been applied as needed ([style="background-color: #888888;"]SSL inspection is required in order to block traffic to sites that use HTTPS)."[/style]

Apart from obvious necessity for a relevant policy rule and security profile -  how does the SSL inspection relate to the DNS filtering?

Regards TK