--- a/sys/bus/u4b/serial/umcs.c
+++ b/sys/bus/u4b/serial/umcs.c
@@ -1058,6 +1058,9 @@
 {
 	uint8_t i = 0;
 
+	if (rate == 0)		/* no div-by-zero, no divisors[9] OOB read */
+		return (-1);
+
 	if (rate > umcs7840_baudrate_divisors[umcs7840_baudrate_divisors_len - 1])
 		return (-1);