diff --git a/lightningd/htlc_set.c b/lightningd/htlc_set.c
index da6604628..402e6b83f 100644
--- a/lightningd/htlc_set.c
+++ b/lightningd/htlc_set.c
@@ -2,6 +2,7 @@
 #include <common/features.h>
 #include <common/timeout.h>
 #include <common/type_to_string.h>
+#include <lightningd/channel.h>
 #include <lightningd/htlc_set.h>
 #include <lightningd/invoice.h>
 #include <lightningd/lightningd.h>
@@ -101,6 +102,7 @@ void htlc_set_add(struct lightningd *ld,
 {
 	struct htlc_set *set;
 	const struct invoice_details *details;
+	const char *err;
 
 	/* BOLT #4:
 	 * The final node:
@@ -109,8 +111,9 @@ void htlc_set_add(struct lightningd *ld,
 	 *     - Note: "amount paid" specified there is the `total_msat` field.
 	 */
 	details = invoice_check_payment(tmpctx, ld, &hin->payment_hash,
-					total_msat, payment_secret);
+					total_msat, payment_secret, &err);
 	if (!details) {
+		log_debug(hin->key.channel->log, "payment failed: %s", err);
 		local_fail_in_htlc(hin,
 				   take(failmsg_incorrect_or_unknown(NULL, ld, hin)));
 		return;
diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index dc086cdd4..512edde95 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -371,7 +371,8 @@ invoice_check_payment(const tal_t *ctx,
 		      struct lightningd *ld,
 		      const struct sha256 *payment_hash,
 		      const struct amount_msat msat,
-		      const struct secret *payment_secret)
+		      const struct secret *payment_secret,
+		      const char **err)
 {
 	u64 inv_dbid;
 	const struct invoice_details *details;
@@ -386,11 +387,12 @@ invoice_check_payment(const tal_t *ctx,
 	 *    - MUST return an `incorrect_or_unknown_payment_details` error.
 	 */
 	if (!invoices_find_unpaid(ld->wallet->invoices, &inv_dbid, payment_hash)) {
-		log_debug(ld->log, "Unknown paid invoice %s",
-			  type_to_string(tmpctx, struct sha256, payment_hash));
 		if (invoices_find_by_rhash(ld->wallet->invoices, &inv_dbid, payment_hash)) {
-			log_debug(ld->log, "ALREADY paid invoice %s",
-				  type_to_string(tmpctx, struct sha256, payment_hash));
+			*err = tal_fmt(ctx, "Already paid or expired invoice %s",
+				       type_to_string(tmpctx, struct sha256, payment_hash));
+		} else {
+			*err = tal_fmt(ctx, "Unknown invoice %s",
+				       type_to_string(tmpctx, struct sha256, payment_hash));
 		}
 		return NULL;
 	}
@@ -405,8 +407,8 @@ invoice_check_payment(const tal_t *ctx,
 	 */
 	if (feature_is_set(details->features, COMPULSORY_FEATURE(OPT_VAR_ONION))
 	    && !payment_secret) {
-		log_debug(ld->log, "Attept to pay %s without secret",
-			  type_to_string(tmpctx, struct sha256, &details->rhash));
+		*err = tal_fmt(ctx, "Attempt to pay %s without secret",
+			       type_to_string(tmpctx, struct sha256, &details->rhash));
 		return tal_free(details);
 	}
 
@@ -418,9 +420,9 @@ invoice_check_payment(const tal_t *ctx,
 		else
 			invoice_secret(&details->r, &expected);
 		if (!secret_eq_consttime(payment_secret, &expected)) {
-			log_debug(ld->log, "Attept to pay %s with wrong secret",
-				  type_to_string(tmpctx, struct sha256,
-						 &details->rhash));
+			*err = tal_fmt(ctx, "Attempt to pay %s with wrong secret",
+				       type_to_string(tmpctx, struct sha256,
+						      &details->rhash));
 			return tal_free(details);
 		}
 	}
@@ -436,21 +438,21 @@ invoice_check_payment(const tal_t *ctx,
 		struct amount_msat twice;
 
 		if (amount_msat_less(msat, *details->msat)) {
-			log_debug(ld->log, "Attept to pay %s with amount %s < %s",
-				  type_to_string(tmpctx, struct sha256,
-						 &details->rhash),
-				  type_to_string(tmpctx, struct amount_msat, &msat),
-				  type_to_string(tmpctx, struct amount_msat, details->msat));
+			*err = tal_fmt(ctx, "Attempt to pay %s with amount %s < %s",
+				       type_to_string(tmpctx, struct sha256,
+						      &details->rhash),
+				       type_to_string(tmpctx, struct amount_msat, &msat),
+				       type_to_string(tmpctx, struct amount_msat, details->msat));
 			return tal_free(details);
 		}
 
 		if (amount_msat_add(&twice, *details->msat, *details->msat)
 		    && amount_msat_greater(msat, twice)) {
-			log_debug(ld->log, "Attept to pay %s with amount %s > %s",
-				  type_to_string(tmpctx, struct sha256,
-						 &details->rhash),
-				  type_to_string(tmpctx, struct amount_msat, &msat),
-				  type_to_string(tmpctx, struct amount_msat, &twice));
+			*err = tal_fmt(ctx, "Attempt to pay %s with amount %s > %s",
+				       type_to_string(tmpctx, struct sha256,
+						      &details->rhash),
+				       type_to_string(tmpctx, struct amount_msat, &msat),
+				       type_to_string(tmpctx, struct amount_msat, &twice));
 			/* BOLT #4:
 			 *
 			 * - if the amount paid is more than twice the amount
diff --git a/lightningd/invoice.h b/lightningd/invoice.h
index 02a3fae77..b167098ca 100644
--- a/lightningd/invoice.h
+++ b/lightningd/invoice.h
@@ -50,6 +50,7 @@ struct invoice_details {
  * @payment_hash: hash of preimage they want.
  * @msat: amount they offer to pay.
  * @payment_secret: they payment secret they sent, if any.
+ * @err: error string if it returns NULL.
  *
  * Returns NULL if there's a problem, otherwise returns the invoice details.
  */
@@ -58,7 +59,8 @@ invoice_check_payment(const tal_t *ctx,
 		      struct lightningd *ld,
 		      const struct sha256 *payment_hash,
 		      const struct amount_msat msat,
-		      const struct secret *payment_secret);
+		      const struct secret *payment_secret,
+		      const char **err);
 
 /**
  * invoice_try_pay - process payment for these incoming payments.
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 63e32d66a..b1cf03e03 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -1916,7 +1916,7 @@ def test_replacement_payload(node_factory):
     with pytest.raises(RpcError, match=r"WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS \(reply from remote\)"):
         l1.rpc.pay(inv)
 
-    assert l2.daemon.wait_for_log("Attept to pay.*with wrong secret")
+    assert l2.daemon.wait_for_log("Attempt to pay.*with wrong secret")
 
 
 @pytest.mark.developer("Requires dev_sign_last_tx")