b.ns.tk has no CNAME records.
Question and response
Question
dig b.ns.tk. NS
Answer
The response has no answer records.
Authority
ns.tk. 5236 SOA a.ns.tk. joost\.zuurbier.dot.tk. 2017063100 14400 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0