diff --git a/guix-data-service/web/compare/controller.scm b/guix-data-service/web/compare/controller.scm
index 20ee3b9..366a0bd 100644
--- a/guix-data-service/web/compare/controller.scm
+++ b/guix-data-service/web/compare/controller.scm
@@ -657,15 +657,18 @@
                  ((name . val)
                   (cons
                    name
-                   (if (invalid-query-parameter? val)
-                       `((invalid
-                          . ,(with-output-to-string
-                               (lambda ()
-                                 (sxml->html
-                                  (invalid-query-parameter-message
-                                   val)))))
-                         (value . ,(invalid-query-parameter-value val)))
-                       val))))
+                   (cond
+                    ((invalid-query-parameter? val)
+                     `((invalid
+                        . ,(with-output-to-string
+                             (lambda ()
+                               (sxml->html
+                                (invalid-query-parameter-message
+                                 val)))))
+                       (value . ,(invalid-query-parameter-value val))))
+                    ((list? val)
+                     (list->vector val))
+                    (else val)))))
                query-parameters)))))
         (else
          (letpar& ((systems