--- a/sys/dev/disk/mpt/mpt.c
+++ b/sys/dev/disk/mpt/mpt.c
@@ -1473,6 +1473,8 @@
 
 	/* Get rest of the reply; but don't overflow the provided buffer */
 	left = (hdr->MsgLength << 1) - 2;
+	if (left < 0)
+		left = 0;
 	reply_left =  reply_len - 2;
 	while (left--) {
 		u_int16_t datum;