Apex domains cannot have CNAME records.
Question and response
Question
dig als-ulm.com. NS
Answer
als-ulm.com. 5 NS ns-927.awsdns-51.net.
als-ulm.com. 5 NS ns-325.awsdns-40.com.
als-ulm.com. 5 NS ns-316.awsdns-39.com.
als-ulm.com. 5 NS ns-1916.awsdns-47.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0