diff --git a/guix/scripts/package.scm b/guix/scripts/package.scm
index b6133b6af0..fa45bd48a6 100644
--- a/guix/scripts/package.scm
+++ b/guix/scripts/package.scm
@@ -486,7 +486,7 @@ Install, remove, or upgrade packages in a single transaction.\n"))
                              arg-handler))))
          (option '(#\u "upgrade") #f #t
                  (lambda (opt name arg result arg-handler)
-                   (when (string-prefix? "-" arg)
+                   (when (and arg (string-prefix? "-" arg))
                      (warning (G_ "upgrade regexp '~a' looks like a \
 command-line option~%")
                               arg)