"invalid input: python"

[Nathan Dehnel <ncdehnel@gmail.com>]

(define-public ladish
  (package
    (name "ladish")
    (version "ladish-1")
    (source
     (origin
       (method git-fetch)
       (uri
        (git-reference
         (url "https://github.com/LADI/ladish.git")
         (commit version)))
       (file-name (git-file-name name version))
       (sha256
        (base32
         "0cmmpz87cs2j15d5jbzffx24gqlgrcq0465k3qx12ai9mbi6lbf0"))))
    (build-system waf-build-system)
    (arguments
      '(#:python ,python-2))
    (inputs
     `(("alsa-lib" ,alsa-lib)
       ("jack2" ,jack-2)
       ("dbus" ,dbus)
       ("e2fsprogs" ,e2fsprogs)
       ("expat" ,expat)
       ("glib" ,glib)
       ("dbus-glib" ,dbus-glib)
       ("gtk+" ,gtk+)))
    (home-page "http://ladish.org/")
    (synopsis "LADI Session Handler")
    (description "LADI Session Handler or simply ladish is a session
management system for JACK applications on GNU/Linux. Its aim is to
allow you to have many different audio programs running at once, to
save their setup, close them down and then easily reload the setup at
some other time. ladish doesn't deal with any kind of audio or MIDI
data itself; it just runs programs, deals with saving/loading
(arbitrary) data and connects JACK ports together. It can also be used
to move entire sessions between computers, or post sessions on the
Internet for download.")
    (license gpl2)))

bash-5.0$ guix package -i ladish
WARNING: (cadence): `expat' imported from both (guix licenses) and
(gnu packages xml)
guix package: error: cadence.scm:142:2: package `ladish@ladish-1' has
an invalid input: ("python" (unquote python-2))

What am I doing wrong? This seems no different from how e.g.
non-sequencer package is doing it.