diff --git a/guix/swh.scm b/guix/swh.scm
index ec744fed2f..913f0d1c9d 100644
--- a/guix/swh.scm
+++ b/guix/swh.scm
@@ -174,7 +174,8 @@ (define string*
   ;; Converts "string or #nil" coming from JSON to "string or #f".
   (match-lambda
     ((? string? str) str)
-    ((? null?) #f)))
+    ((? null?) #f)                                ;Guile-JSON 3.x
+    ('null #f)))                                  ;Guile-JSON 4.x
 
 (define %allow-request?
   ;; Takes a URL and method (e.g., the 'http-get' procedure) and returns true