diff --git a/channeld/channeld.c b/channeld/channeld.c
index 3730a6838..c56735cdc 100644
--- a/channeld/channeld.c
+++ b/channeld/channeld.c
@@ -534,9 +534,12 @@ static struct secret *get_shared_secret(const tal_t *ctx,
 	struct onionpacket *op;
 	struct secret *secret = tal(ctx, struct secret);
 	const u8 *msg;
+	/* FIXME: Use this! */
+	enum onion_type why_bad;
 
 	/* We unwrap the onion now. */
-	op = parse_onionpacket(tmpctx, htlc->routing, TOTAL_PACKET_SIZE);
+	op = parse_onionpacket(tmpctx, htlc->routing, TOTAL_PACKET_SIZE,
+			       &why_bad);
 	if (!op)
 		return tal_free(secret);
 
diff --git a/common/sphinx.c b/common/sphinx.c
index 7bbc4fa4a..95db48663 100644
--- a/common/sphinx.c
+++ b/common/sphinx.c
@@ -75,30 +75,31 @@ u8 *serialize_onionpacket(
 	return dst;
 }
 
-struct onionpacket *parse_onionpacket(
-	const tal_t *ctx,
-	const void *src,
-	const size_t srclen
-	)
+struct onionpacket *parse_onionpacket(const tal_t *ctx,
+				      const void *src,
+				      const size_t srclen,
+				      enum onion_type *why_bad)
 {
 	struct onionpacket *m;
 	int p = 0;
 	u8 rawEphemeralkey[33];
 
-	if (srclen != TOTAL_PACKET_SIZE)
-		return NULL;
+	assert(srclen == TOTAL_PACKET_SIZE);
 
 	m = talz(ctx, struct onionpacket);
 
 	read_buffer(&m->version, src, 1, &p);
 	if (m->version != 0x00) {
 		// FIXME add logging
+		*why_bad = WIRE_INVALID_ONION_VERSION;
 		return tal_free(m);
 	}
 	read_buffer(rawEphemeralkey, src, 33, &p);
 
-	if (secp256k1_ec_pubkey_parse(secp256k1_ctx, &m->ephemeralkey, rawEphemeralkey, 33) != 1)
+	if (secp256k1_ec_pubkey_parse(secp256k1_ctx, &m->ephemeralkey, rawEphemeralkey, 33) != 1) {
+		*why_bad = WIRE_INVALID_ONION_KEY;
 		return tal_free(m);
+	}
 
 	read_buffer(&m->routinginfo, src, ROUTING_INFO_SIZE, &p);
 	read_buffer(&m->mac, src, SECURITY_PARAMETER, &p);
diff --git a/common/sphinx.h b/common/sphinx.h
index 03fdd9350..8a167f06f 100644
--- a/common/sphinx.h
+++ b/common/sphinx.h
@@ -9,6 +9,7 @@
 #include <ccan/tal/tal.h>
 #include <secp256k1.h>
 #include <sodium/randombytes.h>
+#include <wire/gen_onion_wire.h>
 #include <wire/wire.h>
 
 #define SECURITY_PARAMETER 32
@@ -148,13 +149,13 @@ u8 *serialize_onionpacket(
  *
  * @ctx: tal context to allocate from
  * @src: buffer to read the packet from
- * @srclen: length of the @src
+ * @srclen: length of the @src (must be TOTAL_PACKET_SIZE)
+ * @why_bad: if NULL return, this is what was wrong with the packet.
  */
-struct onionpacket *parse_onionpacket(
-	const tal_t *ctx,
-	const void *src,
-	const size_t srclen
-	);
+struct onionpacket *parse_onionpacket(const tal_t *ctx,
+				      const void *src,
+				      const size_t srclen,
+				      enum onion_type *why_bad);
 
 struct onionreply {
 	/* Node index in the path that is replying */
diff --git a/devtools/Makefile b/devtools/Makefile
index e70db606b..4ea586a75 100644
--- a/devtools/Makefile
+++ b/devtools/Makefile
@@ -14,7 +14,8 @@ DEVTOOLS_COMMON_OBJS :=				\
 	common/type_to_string.o			\
 	common/utils.o				\
 	common/version.o			\
-	common/wireaddr.o
+	common/wireaddr.o			\
+	wire/gen_onion_wire.o
 
 devtools-all: $(DEVTOOLS)
 
diff --git a/devtools/onion.c b/devtools/onion.c
index 1237a11d0..33f9e0ed0 100644
--- a/devtools/onion.c
+++ b/devtools/onion.c
@@ -66,6 +66,7 @@ static void do_decode(int argc, char **argv)
 	memset(hextemp, 0, sizeof(hextemp));
 	u8 shared_secret[32];
 	u8 assocdata[32];
+	enum onion_type why_bad;
 
 	memset(&assocdata, 'B', sizeof(assocdata));
 
@@ -82,10 +83,10 @@ static void do_decode(int argc, char **argv)
 		errx(1, "Invalid onion hex '%s'", hextemp);
 	}
 
-	msg = parse_onionpacket(ctx, serialized, sizeof(serialized));
+	msg = parse_onionpacket(ctx, serialized, sizeof(serialized), &why_bad);
 
 	if (!msg)
-		errx(1, "Error parsing message.");
+		errx(1, "Error parsing message: %s", onion_type_name(why_bad));
 
 	if (!onion_shared_secret(shared_secret, msg, &seckey))
 		errx(1, "Error creating shared secret.");
diff --git a/lightningd/peer_htlcs.c b/lightningd/peer_htlcs.c
index 300d3a517..7a9cfac5c 100644
--- a/lightningd/peer_htlcs.c
+++ b/lightningd/peer_htlcs.c
@@ -648,7 +648,8 @@ static bool peer_accepted_htlc(struct channel *channel,
 
 	/* channeld tests this, so it should pass. */
 	op = parse_onionpacket(tmpctx, hin->onion_routing_packet,
-			       sizeof(hin->onion_routing_packet));
+			       sizeof(hin->onion_routing_packet),
+			       failcode);
 	if (!op) {
 		channel_internal_error(channel,
 				       "bad onion in got_revoke: %s",
diff --git a/wallet/test/run-wallet.c b/wallet/test/run-wallet.c
index 063532b9d..b00f65f12 100644
--- a/wallet/test/run-wallet.c
+++ b/wallet/test/run-wallet.c
@@ -367,11 +367,10 @@ struct command_result *param_tok(struct command *cmd UNNEEDED, const char *name
 				 const jsmntok_t **out UNNEEDED)
 { fprintf(stderr, "param_tok called!\n"); abort(); }
 /* Generated stub for parse_onionpacket */
-struct onionpacket *parse_onionpacket(
-	const tal_t *ctx UNNEEDED,
-	const void *src UNNEEDED,
-	const size_t srclen
-	)
+struct onionpacket *parse_onionpacket(const tal_t *ctx UNNEEDED,
+				      const void *src UNNEEDED,
+				      const size_t srclen UNNEEDED,
+				      enum onion_type *why_bad UNNEEDED)
 { fprintf(stderr, "parse_onionpacket called!\n"); abort(); }
 /* Generated stub for payment_failed */
 void payment_failed(struct lightningd *ld UNNEEDED, const struct htlc_out *hout UNNEEDED,