diff --git a/common/bolt11.c b/common/bolt11.c
index 626779c61..f21279c50 100644
--- a/common/bolt11.c
+++ b/common/bolt11.c
@@ -458,6 +458,7 @@ static char *decode_9(struct bolt11 *b11,
                       size_t data_length)
 {
         size_t flen = (data_length * 5 + 7) / 8;
+	int badf;
 
         b11->features = tal_arr(b11, u8, flen);
         pull_bits_certain(hu5, data, data_len, b11->features,
@@ -478,9 +479,9 @@ static char *decode_9(struct bolt11 *b11,
 	 * The field is big-endian.  The least-significant bit is numbered 0,
 	 * which is _even_, and the next most significant bit is numbered 1,
 	 * which is _odd_. */
-	for (size_t i = 0; i < data_length * 5; i += 2)
-		if (feature_is_set(b11->features, i))
-			return tal_fmt(b11, "9: unknown feature bit %zu", i);
+	badf = features_unsupported(b11->features);
+	if (badf != -1)
+		return tal_fmt(b11, "9: unknown feature bit %i", badf);
 
 	return NULL;
 }
diff --git a/common/features.c b/common/features.c
index 8d41a88b0..29f42ae45 100644
--- a/common/features.c
+++ b/common/features.c
@@ -111,8 +111,10 @@ bool feature_negotiated(const u8 *lfeatures, size_t f)
  * @bitmap: the features bitmap the peer is asking for
  * @supported: array of features we support
  * @num_supported: how many elements in supported
+ *
+ * Returns -1 on success, or first unsupported feature.
  */
-static bool all_supported_features(const u8 *bitmap,
+static int all_supported_features(const u8 *bitmap,
 				   const u32 *supported,
 				   size_t num_supported)
 {
@@ -126,18 +128,18 @@ static bool all_supported_features(const u8 *bitmap,
 		if (feature_supported(bitnum, supported, num_supported))
 			continue;
 
-		return false;
+		return bitnum;
 	}
-	return true;
+	return -1;
 }
 
-bool features_supported(const u8 *features)
+int features_unsupported(const u8 *features)
 {
 	/* BIT 2 would logically be "compulsory initial_routing_sync", but
 	 * that does not exist, so we special case it. */
 	if (feature_is_set(features,
 			   COMPULSORY_FEATURE(OPT_INITIAL_ROUTING_SYNC)))
-		return false;
+		return COMPULSORY_FEATURE(OPT_INITIAL_ROUTING_SYNC);
 
 	return all_supported_features(features,
 				      our_features,
diff --git a/common/features.h b/common/features.h
index 8c7a1bb5f..bcb8ccf4c 100644
--- a/common/features.h
+++ b/common/features.h
@@ -4,8 +4,9 @@
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/tal.h>
 
-/* Returns true if we're OK with all these offered features. */
-bool features_supported(const u8 *features);
+/* Returns -1 if we're OK with all these offered features, otherwise first
+ * unsupported (even) feature. */
+int features_unsupported(const u8 *features);
 
 /* For sending our features: tal_count() returns length. */
 u8 *get_offered_features(const tal_t *ctx);
diff --git a/common/test/run-features.c b/common/test/run-features.c
index 81ced7d78..fccbef750 100644
--- a/common/test/run-features.c
+++ b/common/test/run-features.c
@@ -84,17 +84,17 @@ int main(void)
 
 	/* We always support no features. */
 	memset(bits, 0, tal_count(bits));
-	assert(features_supported(bits));
+	assert(features_unsupported(bits) == -1);
 
 	/* We must support our own features. */
 	lf = get_offered_features(tmpctx);
-	assert(features_supported(lf));
+	assert(features_unsupported(lf) == -1);
 
 	/* We can add random odd features, no problem. */
 	for (size_t i = 1; i < 16; i += 2) {
 		bits = tal_dup_arr(tmpctx, u8, lf, tal_count(lf), 0);
 		set_feature_bit(&bits, i);
-		assert(features_supported(bits));
+		assert(features_unsupported(bits) == -1);
 	}
 
 	/* We can't add random even features. */
@@ -104,9 +104,9 @@ int main(void)
 
 		/* Special case for missing compulsory feature */
 		if (i == 2) {
-			assert(!features_supported(bits));
+			assert(features_unsupported(bits) == i);
 		} else {
-			assert(features_supported(bits)
+			assert((features_unsupported(bits) == -1)
 			       == feature_supported(i, our_features,
 						    ARRAY_SIZE(our_features)));
 		}
diff --git a/connectd/peer_exchange_initmsg.c b/connectd/peer_exchange_initmsg.c
index dfbd361f1..014c6b064 100644
--- a/connectd/peer_exchange_initmsg.c
+++ b/connectd/peer_exchange_initmsg.c
@@ -73,7 +73,7 @@ static struct io_plan *peer_init_received(struct io_conn *conn,
 	 *  - upon receiving unknown _even_ feature bits that are non-zero:
 	 *    - MUST fail the connection.
 	 */
-	if (!features_supported(features)) {
+	if (features_unsupported(features) != -1) {
 		const u8 *our_features = get_offered_features(msg);
 		msg = towire_errorfmt(NULL, NULL, "Unsupported features %s:"
 				      " we only offer features %s",