Apex domains cannot have CNAME records.
Question and response
Question
dig v4.enabler.ne.jp. NS
Answer
v4.enabler.ne.jp. 5 NS ns1.enabler.ne.jp.
v4.enabler.ne.jp. 5 NS ns3.enabler.ne.jp.
v4.enabler.ne.jp. 5 NS ns2.enabler.ne.jp.
v4.enabler.ne.jp. 5 NS ns4.enabler.ne.jp.
Authority
The response has no authority records.
Additional
ns1.enabler.ne.jp. 5 A 27.86.3.49
ns2.enabler.ne.jp. 5 A 175.130.241.33
ns3.enabler.ne.jp. 5 A 106.153.224.128
ns4.enabler.ne.jp. 5 A 106.153.224.192
ns1.enabler.ne.jp. 5 AAAA 2001:268:fa04:149:0:0:0:1
ns2.enabler.ne.jp. 5 AAAA 2001:268:fa30:d49:0:0:0:1
ns3.enabler.ne.jp. 5 AAAA 2001:268:fa04:a41:0:0:0:1
ns4.enabler.ne.jp. 5 AAAA 2001:268:fa30:2241:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0