Apex domains cannot have CNAME records.
Question and response
Question
dig gm0.yahoodns.net. NS
Answer
gm0.yahoodns.net. 4 NS yf4.a1.b.yahoo.net.
gm0.yahoodns.net. 4 NS yf2.a1.b.yahoo.net.
gm0.yahoodns.net. 4 NS yf1.a1.b.yahoo.net.
gm0.yahoodns.net. 4 NS yf3.a1.b.yahoo.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0