diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index 82ce069..fdd72b2 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -412,7 +412,9 @@ or is signed by an unauthorized key."
         (and hash signature
              (signature-case (signature hash acl)
                (valid-signature #t)
-               (else #f))))))
+               (else
+                (warning (_ "bad sig!~%"))
+                #f))))))
 
 (define (write-narinfo narinfo port)
   "Write NARINFO to PORT."