nsupdate: SOA RRset prerequisite does not work

Let there be a zone with contents:

example.com.            60      IN      SOA     n71.nebula.cesnet.cz. root.example.com. 20 120 10 3600 60
example.com.            60      NS      n71.nebula.cesnet.cz.

When trying to do a DDNS update using nsupdate utility from BIND, using the SOA rrset as a prerequisite does not work:

$ nsupdate 
> server n71.nebula.cesnet.cz.
> prereq yxrrset example.com. IN SOA n71.nebula.cesnet.cz. root.example.com. 20 120 10 3600 60
> update add test.example.com. 60 IN TXT "TEST"
> show
Outgoing update query:
;; ->>HEADER<<- opcode: UPDATE, status: NOERROR, id:      0
;; flags:; ZONE: 0, PREREQ: 0, UPDATE: 0, ADDITIONAL: 0
;; PREREQUISITE SECTION:
example.com.            0       IN      SOA     n71.nebula.cesnet.cz. root.example.com. 20 120 10 3600 60

;; UPDATE SECTION:
test.example.com.       60      IN      TXT     "TEST"

> send
update failed: NOTZONE