diff --git a/guix/build-system/python.scm b/guix/build-system/python.scm
index 86efc1a715..9a80bd6799 100644
--- a/guix/build-system/python.scm
+++ b/guix/build-system/python.scm
@@ -101,14 +101,11 @@ (define transform
                                                      (string-length old-prefix))
                                           name))))
                (arguments
-                (let ((arguments (package-arguments p))
-                      (python    (if (promise? python)
-                                     (force python)
-                                     python)))
-                  (if (member #:python arguments)
-                      (substitute-keyword-arguments arguments
-                        ((#:python p) python))
-                      (append arguments `(#:python ,python)))))
+                (let ((python (if (promise? python)
+                                  (force python)
+                                  python)))
+                  (ensure-keyword-arguments (package-arguments p)
+                                            `(#:python ,python))))
                (inputs (map rewrite (package-inputs p)))
                (propagated-inputs (map rewrite (package-propagated-inputs p)))
                (native-inputs (map rewrite (package-native-inputs p))))