diff --git a/quic/tools/quic_simple_server_stream.cc b/quic/tools/quic_simple_server_stream.cc
index 4377b56..41fdd6a 100644
--- a/quic/tools/quic_simple_server_stream.cc
+++ b/quic/tools/quic_simple_server_stream.cc
@@ -64,7 +64,7 @@
     SendErrorResponse();
   }
   ConsumeHeaderList();
-  if (!fin) {
+  if (!fin && !response_sent_) {
     // CONNECT and other CONNECT-like methods (such as CONNECT-UDP) require
     // sending the response right after parsing the headers even though the FIN
     // bit has not been received on the request stream.
@@ -290,6 +290,8 @@
         quiche::QuicheTextUtils::Uint64ToString(generate_bytes_length_);
 
     WriteHeaders(std::move(headers), false, nullptr);
+    QUICHE_DCHECK(!response_sent_);
+    response_sent_ = true;
 
     WriteGeneratedBytes();
 
@@ -349,6 +351,8 @@
   QUIC_DLOG(INFO) << "Stream " << id() << " writing headers (fin = false) : "
                   << response_headers.DebugString();
   WriteHeaders(std::move(response_headers), /*fin=*/false, nullptr);
+  QUICHE_DCHECK(!response_sent_);
+  response_sent_ = true;
 
   QUIC_DLOG(INFO) << "Stream " << id()
                   << " writing body (fin = false) with size: " << body.size();
@@ -373,6 +377,8 @@
   QUIC_DLOG(INFO) << "Stream " << id() << " writing headers (fin = " << send_fin
                   << ") : " << response_headers.DebugString();
   WriteHeaders(std::move(response_headers), send_fin, nullptr);
+  QUICHE_DCHECK(!response_sent_);
+  response_sent_ = true;
   if (send_fin) {
     // Nothing else to send.
     return;
diff --git a/quic/tools/quic_simple_server_stream.h b/quic/tools/quic_simple_server_stream.h
index bbed95a..2dd59f9 100644
--- a/quic/tools/quic_simple_server_stream.h
+++ b/quic/tools/quic_simple_server_stream.h
@@ -99,6 +99,8 @@
 
  private:
   uint64_t generate_bytes_length_;
+  // Whether response headers have already been sent.
+  bool response_sent_ = false;
 
   QuicSimpleServerBackend* quic_simple_server_backend_;  // Not owned.
 };
diff --git a/quic/tools/quic_simple_server_stream_test.cc b/quic/tools/quic_simple_server_stream_test.cc
index 541316a..13b0335 100644
--- a/quic/tools/quic_simple_server_stream_test.cc
+++ b/quic/tools/quic_simple_server_stream_test.cc
@@ -790,6 +790,32 @@
   EXPECT_FALSE(stream_->send_error_response_was_called());
 }
 
+TEST_P(QuicSimpleServerStreamTest, ConnectWithInvalidHeader) {
+  EXPECT_CALL(session_, WritevData(_, _, _, _, _, _))
+      .WillRepeatedly(
+          Invoke(&session_, &MockQuicSimpleServerSession::ConsumeData));
+  QuicHeaderList header_list;
+  header_list.OnHeaderBlockStart();
+  header_list.OnHeader(":authority", "www.google.com:4433");
+  header_list.OnHeader(":method", "CONNECT-SILLY");
+  // QUIC requires lower-case header names.
+  header_list.OnHeader("InVaLiD-HeAdEr", "Well that's just wrong!");
+  header_list.OnHeaderBlockEnd(128, 128);
+  EXPECT_CALL(*stream_, WriteHeadersMock(/*fin=*/false));
+  stream_->OnStreamHeaderList(/*fin=*/false, kFakeFrameLen, header_list);
+  std::unique_ptr<char[]> buffer;
+  QuicByteCount header_length =
+      HttpEncoder::SerializeDataFrameHeader(body_.length(), &buffer);
+  std::string header = std::string(buffer.get(), header_length);
+  std::string data = UsesHttp3() ? header + body_ : body_;
+  stream_->OnStreamFrame(
+      QuicStreamFrame(stream_->id(), /*fin=*/false, /*offset=*/0, data));
+  EXPECT_EQ("CONNECT-SILLY", StreamHeadersValue(":method"));
+  EXPECT_EQ(body_, StreamBody());
+  EXPECT_FALSE(stream_->send_response_was_called());
+  EXPECT_TRUE(stream_->send_error_response_was_called());
+}
+
 }  // namespace
 }  // namespace test
 }  // namespace quic