diff --git a/usr.bin/m4/gnum4.c b/usr.bin/m4/gnum4.c
index 5ff289a9fae..2ba45f26182 100644
--- a/usr.bin/m4/gnum4.c
+++ b/usr.bin/m4/gnum4.c
@@ -279,7 +279,7 @@ add_sub(n, string, re, pm)
 	regmatch_t *pm;
 {
 	if (n > re->re_nsub)
-		warnx("No subexpression %d", n);
+		warnx("No subexpression %u", n);
 	/* Subexpressions that did not match are
 	 * not an error.  */
 	else if (pm[n].rm_so != -1 &&