paymentsdb: verify total amount for last hop in the blinded path

diff --git a/payments/db/errors.go b/payments/db/errors.go
index fee71b0..0457db6 100644
--- a/payments/db/errors.go
+++ b/payments/db/errors.go
@@ -84,6 +84,12 @@ var (
 	ErrMixedBlindedAndNonBlindedPayments = errors.New("mixed blinded and " +
 		"non-blinded payments")
 
+	// ErrBlindedPaymentMissingTotalAmount is returned if we try to
+	// register a blinded payment attempt where the final hop doesn't set
+	// the total amount.
+	ErrBlindedPaymentMissingTotalAmount = errors.New("blinded payment " +
+		"final hop must set total amount")
+
 	// ErrMPPPaymentAddrMismatch is returned if we try to register an MPP
 	// shard where the payment address doesn't match existing shards.
 	ErrMPPPaymentAddrMismatch = errors.New("payment address mismatch")
diff --git a/payments/db/payment.go b/payments/db/payment.go
index 147ccdb..ddceedf 100644
--- a/payments/db/payment.go
+++ b/payments/db/payment.go
@@ -744,6 +744,13 @@ func verifyAttempt(payment *MPPayment, attempt *HTLCAttemptInfo) error {
 	// in the split payment is correct.
 	isBlinded := len(attempt.Route.FinalHop().EncryptedData) != 0
 
+	// For blinded payments, the last hop must set the total amount.
+	if isBlinded {
+		if attempt.Route.FinalHop().TotalAmtMsat == 0 {
+			return ErrBlindedPaymentMissingTotalAmount
+		}
+	}
+
 	// Make sure any existing shards match the new one with regards
 	// to MPP options.
 	mpp := attempt.Route.FinalHop().MPP
diff --git a/payments/db/payment_test.go b/payments/db/payment_test.go
index a7369c1..e6a2e73 100644
--- a/payments/db/payment_test.go
+++ b/payments/db/payment_test.go
@@ -1388,6 +1388,45 @@ func TestVerifyAttemptBlindedValidation(t *testing.T) {
 	require.NoError(t, verifyAttempt(payment, &matching))
 }
 
+// TestVerifyAttemptBlindedMissingTotalAmount tests that we return an error if
+// we try to register a blinded payment attempt where the final hop doesn't set
+// the total amount.
+func TestVerifyAttemptBlindedMissingTotalAmount(t *testing.T) {
+	t.Parallel()
+
+	total := lnwire.MilliSatoshi(5000)
+
+	// Payment with no existing attempts.
+	payment := makePayment(total)
+
+	// Attempt with encrypted data (blinded payment) but missing total
+	// amount.
+	attemptMissingTotal := makeLastHopAttemptInfo(
+		1,
+		lastHopArgs{
+			amt:       2500,
+			total:     0,
+			encrypted: []byte{1, 2, 3},
+		},
+	)
+	require.ErrorIs(
+		t,
+		verifyAttempt(payment, &attemptMissingTotal),
+		ErrBlindedPaymentMissingTotalAmount,
+	)
+
+	// Attempt with encrypted data and valid total amount should succeed.
+	attemptWithTotal := makeLastHopAttemptInfo(
+		2,
+		lastHopArgs{
+			amt:       2500,
+			total:     total,
+			encrypted: []byte{4, 5, 6},
+		},
+	)
+	require.NoError(t, verifyAttempt(payment, &attemptWithTotal))
+}
+
 // TestVerifyAttemptBlindedMixedWithNonBlinded tests that we return an error if
 // we try to register a non-MPP attempt for a blinded payment.
 func TestVerifyAttemptBlindedMixedWithNonBlinded(t *testing.T) {