Wrong type argument in position 1 (expecting struct): #<procedure sddm-service (#:optional config)>

[znavko]

Hello! I try to reconfigure system with Gnome, sddm and wayland.
I also use sddm-configuration to define use wayland for greeter, but get the 
error for my misconfiguration:

# cat config.scm
...
 (services (cons*
 ;(service xfce-desktop-service-type)
 (service gnome-desktop-service-type)
 (service dhcp-client-service-type)
 (service sddm-service
 (sddm-configuration
 (display-server "wayland")))
...
)

# guix system reconfigure config.scm
Backtrace:
 14 (primitive-load "/root/.config/guix/current/bin/guix")
In guix/ui.scm:
 1734:12 13 (run-guix-command _ . _)
In ice-9/boot-9.scm:
 829:9 12 (catch _ _ #<procedure 7ff6047408f8 at guix/ui.scm:701…> …)
 829:9 11 (catch _ _ #<procedure 7ff604740910 at guix/ui.scm:824…> …)
In guix/scripts/system.scm:
 1301:8 10 (_)
In guix/status.scm:
 768:4 9 (call-with-status-report _ _)
In guix/scripts/system.scm:
 1159:4 8 (process-action _ _ _)
In guix/store.scm:
 623:10 7 (call-with-store _)
 1794:24 6 (run-with-store _ _ #:guile-for-build _ #:system _ # _)
In guix/scripts/system.scm:
 1175:13 5 (_ _)
 882:18 4 (perform-action reconfigure #<<operating-system> kerne…> …)
In gnu/system.scm:
 851:19 3 (operating-system-derivation _)
In gnu/services.scm:
 743:6 2 (instantiate-missing-services _)
In guix/combinators.scm:
 45:26 1 (fold2 #<procedure adjust-service-list (svc result ins…> …)
In gnu/services.scm:
 733:11 0 (adjust-service-list _ (#<<service> type: #<service…> …) …)

gnu/services.scm:733:11: In procedure adjust-service-list:
In procedure struct_vtable: Wrong type argument in position 1 (expecting 
struct): #<procedure sddm-service (#:optional config)>
Please, let me know how to understand such errors. Here, I saw the same 
structure 
https://www.gnu.org/software/guix/manual/en/guix.html#index-sddm_002dservice

config-znavko-gnome.scm
Description: Binary data