diff --git a/guix/gnu-maintenance.scm b/guix/gnu-maintenance.scm
index d8b6af9d31..7b608daea2 100644
--- a/guix/gnu-maintenance.scm
+++ b/guix/gnu-maintenance.scm
@@ -176,7 +176,7 @@ (define (mirror-type url)
 
        (let ((url  (and=> (package-source package) origin-uri))
              (name (package-name package)))
-         (case (and url (mirror-type url))
+         (case (and (string? url) (mirror-type url))
            ((gnu) #t)
            ((non-gnu) #f)
            (else