diff --git a/bin/ps/fmt.c b/bin/ps/fmt.c
index 92638869227d..3703ace22c6e 100644
--- a/bin/ps/fmt.c
+++ b/bin/ps/fmt.c
@@ -70,7 +70,8 @@ shquote(char **argv)
 	if (buf == NULL) {
 		if ((arg_max = sysconf(_SC_ARG_MAX)) == -1)
 			errx(1, "sysconf _SC_ARG_MAX failed");
-		if (arg_max >= LONG_MAX / 4 || 4 * arg_max + 1 > SIZE_MAX)
+		if (arg_max >= LONG_MAX / 4 ||
+		    4 * (size_t)arg_max + 1 > SIZE_MAX)
 			errx(1, "sysconf _SC_ARG_MAX preposterously large");
 		buf_size = 4 * arg_max + 1;
 		if ((buf = malloc(buf_size)) == NULL)