diff --git a/pc/jsep_transport_controller.cc b/pc/jsep_transport_controller.cc
index 792365b521..fdf6598eea 100644
--- a/pc/jsep_transport_controller.cc
+++ b/pc/jsep_transport_controller.cc
@@ -641,7 +641,12 @@ RTCError JsepTransportController::ApplyDescription_n(
 
     cricket::JsepTransport* transport =
         GetJsepTransportForMid(content_info.name);
-    RTC_DCHECK(transport);
+    if (!transport) {
+      LOG_AND_RETURN_ERROR(
+          RTCErrorType::INVALID_PARAMETER,
+          "Could not find transport for m= section with mid='" +
+              content_info.name + "'");
+    }
 
     SetIceRole_n(DetermineIceRole(transport, transport_info, type, local));
 
diff --git a/pc/sdp_offer_answer_unittest.cc b/pc/sdp_offer_answer_unittest.cc
index 03bea2a23e..14e9557ca6 100644
--- a/pc/sdp_offer_answer_unittest.cc
+++ b/pc/sdp_offer_answer_unittest.cc
@@ -959,4 +959,38 @@ TEST_F(SdpOfferAnswerTest, OfferWithRtxAndNoMsidIsNotRejected) {
   EXPECT_TRUE(pc->SetRemoteDescription(std::move(offer)));
 }
 
+TEST_F(SdpOfferAnswerTest, RejectsAnswerWithInvalidTransport) {
+  auto pc1 = CreatePeerConnection();
+  pc1->AddAudioTrack("audio_track", {});
+  auto pc2 = CreatePeerConnection();
+  pc2->AddAudioTrack("anotheraudio_track", {});
+
+  auto initial_offer = pc1->CreateOfferAndSetAsLocal();
+  ASSERT_EQ(initial_offer->description()->contents().size(), 1u);
+  auto mid = initial_offer->description()->contents()[0].mid();
+
+  EXPECT_TRUE(pc2->SetRemoteDescription(std::move(initial_offer)));
+  auto initial_answer = pc2->CreateAnswerAndSetAsLocal();
+
+  std::string sdp;
+  initial_answer->ToString(&sdp);
+  EXPECT_TRUE(pc1->SetRemoteDescription(std::move(initial_answer)));
+
+  auto transceivers = pc1->pc()->GetTransceivers();
+  ASSERT_EQ(transceivers.size(), 1u);
+  // This stops the only transport.
+  transceivers[0]->StopStandard();
+
+  auto subsequent_offer = pc1->CreateOfferAndSetAsLocal();
+  // But the remote answers with a non-rejected m-line which is not valid.
+  auto bad_answer = CreateSessionDescription(
+      SdpType::kAnswer,
+      absl::StrReplaceAll(sdp, {{"a=group:BUNDLE " + mid + "\r\n", ""}}));
+
+  RTCError error;
+  pc1->SetRemoteDescription(std::move(bad_answer), &error);
+  EXPECT_FALSE(error.ok());
+  EXPECT_EQ(error.type(), RTCErrorType::INVALID_PARAMETER);
+}
+
 }  // namespace webrtc