diff --git a/common/message.c b/common/message.c
index 94474253be..6828273e78 100644
--- a/common/message.c
+++ b/common/message.c
@@ -69,7 +69,7 @@ int message_process_cmd(int cmd, uint8_t *out_msg, int max_len)
 	 */
 	if (msg_len + MSG_PROTO_BYTES > max_len)
 		msg_len = max_len - MSG_PROTO_BYTES;
-	len = msg_len + 4;
+	len = msg_len + MSG_PROTO_BYTES;
 	ASSERT(msg_len >= 0 && msg_len < 0xffff);
 	need_copy = msg != out_msg + MSG_HEADER_BYTES;
 	ASSERT(!need_copy ||
diff --git a/include/message.h b/include/message.h
index 867dff8b58..fa41d9597c 100644
--- a/include/message.h
+++ b/include/message.h
@@ -46,8 +46,8 @@ enum message_cmd_t {
  * The hdr byte is just a tag to indicate that the real message follows. It
  * signals the end of any preamble required by the interface.
  *
- * The 16-bit length includes the length of the header, len bytes, message
- * and checksum, but not the final preamble byte.
+ * The 16-bit length is the entire packet size, including the header, length
+ * bytes, message payload, checksum, and postamble byte.
  *
  * The checksum is calculated as the sum of the header, len byte and message.
  *