Apex domains cannot have CNAME records.
Question and response
Question
dig uptimerobot.com. NS
Answer
uptimerobot.com. 5 NS cruz.ns.cloudflare.com.
uptimerobot.com. 5 NS darwin.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0