Apex domains cannot have CNAME records.
Question and response
Question
dig sigma-serv.com. NS
Answer
sigma-serv.com. 4 NS ns2.westcon.com.
sigma-serv.com. 4 NS ns1.westcon.co.uk.
sigma-serv.com. 4 NS ns1.westcon.com.
sigma-serv.com. 4 NS ns4.westcon.com.
sigma-serv.com. 4 NS ns3.westcon.com.
Authority
The response has no authority records.
Additional
ns1.westcon.co.uk. 4 A 216.150.156.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0