Apex domains cannot have CNAME records.
Question and response
Question
dig lightning.net. NS
Answer
lightning.net. 5 NS storm.lightning.net.
lightning.net. 5 NS thunder.lightning.net.
Authority
The response has no authority records.
Additional
storm.lightning.net. 5 A 209.51.180.102
thunder.lightning.net. 5 A 209.51.160.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0