Apex domains cannot have CNAME records.
Question and response
Question
dig dendrite.com. NS
Answer
dendrite.com. 5 NS dnsus1.imshealth.com.
dendrite.com. 5 NS dnsuk.imshealth.com.
dendrite.com. 5 NS dnsuk2.imshealth.com.
dendrite.com. 5 NS dnsus2.imshealth.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0