Apex domains cannot have CNAME records.
Question and response
Question
dig 17.156.IN-ADDR.ARPA. NS
Answer
17.156.IN-ADDR.ARPA. 3 NS wask.wask.wroc.pl.
17.156.IN-ADDR.ARPA. 3 NS sun2.pwr.wroc.pl.
17.156.IN-ADDR.ARPA. 3 NS ns1.net.icm.edu.pl.
17.156.IN-ADDR.ARPA. 3 NS ldhpux.immt.pwr.wroc.pl.
Authority
The response has no authority records.
Additional
sun2.pwr.wroc.pl. 3 A 156.17.5.2
wask.wask.wroc.pl. 3 A 156.17.254.3
ldhpux.immt.pwr.wroc.pl. 3 A 156.17.8.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0