diff --git a/doc/ethapp.asc b/doc/ethapp.asc
index c22034f..7ae5db3 100644
--- a/doc/ethapp.asc
+++ b/doc/ethapp.asc
@@ -168,6 +168,7 @@ The input data is the message to sign, streamed to the device in 255 bytes maxim
 | First derivation index (big endian)                                               | 4
 | ...                                                                               | 4
 | Last derivation index (big endian)                                                | 4
+| Message length                                                                    | 4
 | Message chunk                                                                     | variable
 |==============================================================================================================================