Apex domains cannot have CNAME records.
Question and response
Question
dig sparkasse.de. NS
Answer
sparkasse.de. 5 NS ns3.s-fg-net.eu.
sparkasse.de. 5 NS ns2.s-fg-net.com.
sparkasse.de. 5 NS ns1.s-fg-net.de.
sparkasse.de. 5 NS ns4.s-fg-net.de.
Authority
The response has no authority records.
Additional
ns1.s-fg-net.de. 5 A 62.181.159.76
ns2.s-fg-net.com. 5 A 195.140.118.140
ns3.s-fg-net.eu. 5 A 62.181.159.86
ns4.s-fg-net.de. 5 A 195.140.118.150
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0