diff --git a/guix/store.scm b/guix/store.scm
index 45c555b..35cb680 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -487,7 +487,7 @@ encoding conversion errors."
       (let ((pairs `(,@(if timeout
                            `(("build-timeout" . ,(number->string timeout)))
                            '())
-                     ("substitute-urls" . ,(string-join substitute-urls)))))
+                     ("substitute-urls" . ,(pk 'surls (string-join substitute-urls))))))
         (send (string-pairs pairs))))
     (let loop ((done? (process-stderr server)))
       (or done? (process-stderr server)))))