diff --git a/src/session.rs b/src/session.rs
index a93203dc..dcbe9c6f 100644
--- a/src/session.rs
+++ b/src/session.rs
@@ -706,7 +706,13 @@ impl Session {
 
         let params = &self.codec_config;
         let exts = media.remote_extmap();
-        let receipt = stream.poll_packet(now, exts, &mut self.twcc, params, buf)?;
+
+        // TWCC might not be enabled for this m-line. Firefox do use TWCC, but not
+        // for audio. This is indiciated via the SDP.
+        let twcc_enabled = exts.id_of(Extension::TransportSequenceNumber).is_some();
+        let twcc = twcc_enabled.then_some(&mut self.twcc);
+
+        let receipt = stream.poll_packet(now, exts, twcc, params, buf)?;
 
         let PacketReceipt {
             header,
@@ -732,7 +738,7 @@ impl Session {
 
         let protected = srtp_tx.protect_rtp(buf, &header, *seq_no);
 
-        if exts.id_of(Extension::TransportSequenceNumber).is_some() {
+        if twcc_enabled {
             self.twcc_tx_register
                 .register_seq(twcc_seq.into(), now, payload_size);
         }
diff --git a/src/streams/send.rs b/src/streams/send.rs
index 0995a61d..19be1222 100644
--- a/src/streams/send.rs
+++ b/src/streams/send.rs
@@ -14,7 +14,6 @@ use crate::packet::QueuePriority;
 use crate::packet::QueueSnapshot;
 use crate::packet::QueueState;
 use crate::rtp_::Bitrate;
-use crate::rtp_::Extension;
 use crate::rtp_::{extend_u16, Descriptions, ReportList, Rtcp};
 use crate::rtp_::{ExtensionMap, ReceptionReport, RtpHeader};
 use crate::rtp_::{ExtensionValues, Frequency, MediaTime, Mid, NackEntry};
@@ -362,7 +361,7 @@ impl StreamTx {
         &mut self,
         now: Instant,
         exts: &ExtensionMap,
-        twcc: &mut u64,
+        twcc: Option<&mut u64>,
         params: &[PayloadParams],
         buf: &mut Vec<u8>,
     ) -> Option<PacketReceipt> {
@@ -467,7 +466,9 @@ impl StreamTx {
         // These need to match `Extension::is_supported()` so we are sending what we are
         // declaring we support.
         header.ext_vals.abs_send_time = Some(now);
-        if exts.id_of(Extension::TransportSequenceNumber).is_some() {
+
+        // TWCC might not be enabled for this m-line.
+        if let Some(twcc) = twcc {
             header.ext_vals.transport_cc = Some(*twcc as u16);
             *twcc += 1;
         }