diff --git a/lightning/src/ln/onion_utils.rs b/lightning/src/ln/onion_utils.rs
index 5ad68407631..0a730ef950b 100644
--- a/lightning/src/ln/onion_utils.rs
+++ b/lightning/src/ln/onion_utils.rs
@@ -418,12 +418,20 @@ pub(super) fn construct_onion_packet(
 #[allow(unused)]
 pub(super) fn construct_trampoline_onion_packet(
 	payloads: Vec<msgs::OutboundTrampolinePayload>, onion_keys: Vec<OnionKeys>,
-	prng_seed: [u8; 32], associated_data: &PaymentHash, length: u16,
+	prng_seed: [u8; 32], associated_data: &PaymentHash,
 ) -> Result<msgs::TrampolineOnionPacket, ()> {
-	let mut packet_data = vec![0u8; length as usize];
+	let packet_length: usize = payloads
+		.iter()
+		.map(|p| {
+			let mut payload_len = LengthCalculatingWriter(0);
+			p.write(&mut payload_len).expect("Failed to calculate length");
+			payload_len.0 + 32
+		})
+		.sum();
 
+	let mut packet_data = vec![0u8; packet_length];
 	let mut chacha = ChaCha20::new(&prng_seed, &[0; 8]);
-	chacha.process(&vec![0u8; length as usize], &mut packet_data);
+	chacha.process(&vec![0u8; packet_length], &mut packet_data);
 
 	construct_onion_packet_with_init_noise::<_, _>(
 		payloads,