Apex domains cannot have CNAME records.
Question and response
Question
dig guidecom.de. NS
Answer
guidecom.de. 3 NS ns.ui-global-dns.com.
guidecom.de. 3 NS ns.ui-global-dns.de.
guidecom.de. 3 NS ns.ui-global-dns.biz.
guidecom.de. 3 NS ns.ui-global-dns.org.
Authority
The response has no authority records.
Additional
ns.ui-global-dns.de. 3 A 192.174.68.90
ns.ui-global-dns.biz. 3 A 192.174.68.7
ns.ui-global-dns.com. 3 A 176.97.158.7
ns.ui-global-dns.org. 3 A 176.97.158.90
ns.ui-global-dns.de. 3 AAAA 2001:67c:1bc:0:0:0:0:90
ns.ui-global-dns.biz. 3 AAAA 2001:67c:1bc:0:0:0:0:7
ns.ui-global-dns.com. 3 AAAA 2001:67c:10b8:0:0:0:0:7
ns.ui-global-dns.org. 3 AAAA 2001:67c:10b8:0:0:0:0:90
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0