Last session id is only sent optionally, it can be omitted if not available.

docs/pokerth.asn1

@@ -96,7 +96,7 @@ AnnounceMessage ::= [APPLICATION 0] SEQUENCE {
 InitMessage ::= [APPLICATION 1] SEQUENCE {
 	requestedVersion			Version,
 	buildId						INTEGER,
-	myLastSessionId				Guid,
+	myLastSessionId				Guid OPTIONAL,
 	login						CHOICE {
 		guestLogin				[0]	GuestLogin,
 		authenticatedLogin		[1]	AuthenticatedLogin,

src/third_party/asn1/InitMessage.c

@@ -93,7 +93,7 @@ static asn_TYPE_member_t asn_MBR_InitMessage_1[] = {
 		0,
 		"buildId"
 	},
-	{	ATF_NOFLAGS, 0, offsetof(struct InitMessage, myLastSessionId),
+	{	ATF_POINTER, 1, offsetof(struct InitMessage, myLastSessionId),
 		(ASN_TAG_CLASS_UNIVERSAL | (4 << 2)),
 		0,
 		&asn_DEF_Guid,

src/third_party/asn1/InitMessage.h

@@ -40,7 +40,7 @@ extern "C" {
 	typedef struct InitMessage {
 		Version_t	 requestedVersion;
 		long	 buildId;
-		Guid_t	 myLastSessionId;
+		Guid_t	*myLastSessionId	/* OPTIONAL */;
 		struct login {
 			login_PR present;
 			union InitMessage__login_u {