diff --git a/drivers/s390/char/sclp_async.c b/drivers/s390/char/sclp_async.c
index 2aecf7f21361b9d430b531e9b6697a8515a84923..7ad30e72f868de1e45ef7e4bdd81cfa8a4701ca4 100644
--- a/drivers/s390/char/sclp_async.c
+++ b/drivers/s390/char/sclp_async.c
@@ -85,7 +85,7 @@ static int proc_handler_callhome(struct ctl_table *ctl, int write,
 		rc = copy_from_user(buf, buffer, sizeof(buf));
 		if (rc != 0)
 			return -EFAULT;
-		buf[len - 1] = '\0';
+		buf[sizeof(buf) - 1] = '\0';
 		if (strict_strtoul(buf, 0, &val) != 0)
 			return -EINVAL;
 		if (val != 0 && val != 1)