# why does adding a DNS root zone to the URL sometimes result in security errors?

Disclaimer: I've only witnessed this behaviour on chrome. adding the rightmost dot to a URL as in facebook.com. should result in a fully qualified domain name (FQDN) and hence I wouldn't expect any problems when resolving that url. However chrome throws a security error the first time you're trying to access it.

is there an explanation for this error?

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Facebook uses by default HTTPS, the SSL certificate facebook uses at https://www.facebook.com/ has a CN (Common Name) of www.facebook.com not www.facebook.com..
Or maybe you didn't notice that once you accepted it, facebook will actually redirect www.facebook.com. to www.facebook.com.