diff --git a/quic/core/http/quic_spdy_stream.cc b/quic/core/http/quic_spdy_stream.cc
index afeeef0..5317137 100644
--- a/quic/core/http/quic_spdy_stream.cc
+++ b/quic/core/http/quic_spdy_stream.cc
@@ -185,6 +185,8 @@
       headers_payload_length_(0),
       trailers_decompressed_(false),
       trailers_consumed_(false),
+      qpack_decoded_headers_accumulator_reset_reason_(
+          QpackDecodedHeadersAccumulatorResetReason::kUnSet),
       http_decoder_visitor_(std::make_unique<HttpDecoderVisitor>(this)),
       decoder_(http_decoder_visitor_.get(),
                HttpDecoderOptionsForBidiStream(spdy_session)),
@@ -225,6 +227,8 @@
       headers_payload_length_(0),
       trailers_decompressed_(false),
       trailers_consumed_(false),
+      qpack_decoded_headers_accumulator_reset_reason_(
+          QpackDecodedHeadersAccumulatorResetReason::kUnSet),
       http_decoder_visitor_(std::make_unique<HttpDecoderVisitor>(this)),
       decoder_(http_decoder_visitor_.get()),
       sequencer_offset_(sequencer()->NumBytesConsumed()),
@@ -563,6 +567,8 @@
                                       bool header_list_size_limit_exceeded) {
   header_list_size_limit_exceeded_ = header_list_size_limit_exceeded;
   qpack_decoded_headers_accumulator_.reset();
+  qpack_decoded_headers_accumulator_reset_reason_ =
+      QpackDecodedHeadersAccumulatorResetReason::kResetInOnHeadersDecoded;
 
   QuicSpdySession::LogHeaderCompressionRatioHistogram(
       /* using_qpack = */ true,
@@ -592,6 +598,8 @@
 void QuicSpdyStream::OnHeaderDecodingError(QuicErrorCode error_code,
                                            absl::string_view error_message) {
   qpack_decoded_headers_accumulator_.reset();
+  qpack_decoded_headers_accumulator_reset_reason_ =
+      QpackDecodedHeadersAccumulatorResetReason::kResetInOnHeaderDecodingError;
 
   std::string connection_close_error_message = absl::StrCat(
       "Error decoding ", headers_decompressed_ ? "trailers" : "headers",
@@ -756,6 +764,8 @@
       if (GetQuicReloadableFlag(quic_abort_qpack_on_stream_reset)) {
         QUIC_RELOADABLE_FLAG_COUNT_N(quic_abort_qpack_on_stream_reset, 1, 2);
         qpack_decoded_headers_accumulator_.reset();
+        qpack_decoded_headers_accumulator_reset_reason_ =
+            QpackDecodedHeadersAccumulatorResetReason::kResetInOnStreamReset1;
       }
     }
 
@@ -769,6 +779,8 @@
     if (!fin_received() && spdy_session_->qpack_decoder()) {
       spdy_session_->qpack_decoder()->OnStreamReset(id());
       qpack_decoded_headers_accumulator_.reset();
+      qpack_decoded_headers_accumulator_reset_reason_ =
+          QpackDecodedHeadersAccumulatorResetReason::kResetInOnStreamReset2;
     }
 
     QuicStream::OnStreamReset(frame);
@@ -791,6 +803,8 @@
     if (GetQuicReloadableFlag(quic_abort_qpack_on_stream_reset)) {
       QUIC_RELOADABLE_FLAG_COUNT_N(quic_abort_qpack_on_stream_reset, 2, 2);
       qpack_decoded_headers_accumulator_.reset();
+      qpack_decoded_headers_accumulator_reset_reason_ =
+          QpackDecodedHeadersAccumulatorResetReason::kResetInResetWithError;
     }
   }
 
@@ -893,6 +907,8 @@
   QuicStream::OnClose();
 
   qpack_decoded_headers_accumulator_.reset();
+  qpack_decoded_headers_accumulator_reset_reason_ =
+      QpackDecodedHeadersAccumulatorResetReason::kResetInOnClose;
 
   if (visitor_) {
     Visitor* visitor = visitor_;
@@ -1088,7 +1104,14 @@
 
 bool QuicSpdyStream::OnHeadersFramePayload(absl::string_view payload) {
   QUICHE_DCHECK(VersionUsesHttp3(transport_version()));
-  QUICHE_DCHECK(qpack_decoded_headers_accumulator_);
+
+  if (!qpack_decoded_headers_accumulator_) {
+    QUIC_BUG(b215142466_OnHeadersFramePayload)
+        << static_cast<int>(qpack_decoded_headers_accumulator_reset_reason_);
+    OnHeaderDecodingError(QUIC_INTERNAL_ERROR,
+                          "qpack_decoded_headers_accumulator_ is nullptr");
+    return false;
+  }
 
   qpack_decoded_headers_accumulator_->Decode(payload);
 
@@ -1103,7 +1126,14 @@
 
 bool QuicSpdyStream::OnHeadersFrameEnd() {
   QUICHE_DCHECK(VersionUsesHttp3(transport_version()));
-  QUICHE_DCHECK(qpack_decoded_headers_accumulator_);
+
+  if (!qpack_decoded_headers_accumulator_) {
+    QUIC_BUG(b215142466_OnHeadersFrameEnd)
+        << static_cast<int>(qpack_decoded_headers_accumulator_reset_reason_);
+    OnHeaderDecodingError(QUIC_INTERNAL_ERROR,
+                          "qpack_decoded_headers_accumulator_ is nullptr");
+    return false;
+  }
 
   qpack_decoded_headers_accumulator_->EndHeaderBlock();
 
diff --git a/quic/core/http/quic_spdy_stream.h b/quic/core/http/quic_spdy_stream.h
index 8ec56d5..14ac2f2 100644
--- a/quic/core/http/quic_spdy_stream.h
+++ b/quic/core/http/quic_spdy_stream.h
@@ -391,6 +391,20 @@
     WebTransportStreamAdapter adapter;
   };
 
+  // Reason codes for `qpack_decoded_headers_accumulator_` being nullptr.
+  enum class QpackDecodedHeadersAccumulatorResetReason {
+    // `qpack_decoded_headers_accumulator_` was default constructed to nullptr.
+    kUnSet = 0,
+    // `qpack_decoded_headers_accumulator_` was reset in the corresponding
+    // method.
+    kResetInOnHeadersDecoded = 1,
+    kResetInOnHeaderDecodingError = 2,
+    kResetInOnStreamReset1 = 3,
+    kResetInOnStreamReset2 = 4,
+    kResetInResetWithError = 5,
+    kResetInOnClose = 6,
+  };
+
   // Called by HttpDecoderVisitor.
   bool OnDataFrameStart(QuicByteCount header_length,
                         QuicByteCount payload_length);
@@ -462,6 +476,9 @@
   // Headers accumulator for decoding HEADERS frame payload.
   std::unique_ptr<QpackDecodedHeadersAccumulator>
       qpack_decoded_headers_accumulator_;
+  // Reason for `qpack_decoded_headers_accumulator_` being nullptr.
+  QpackDecodedHeadersAccumulatorResetReason
+      qpack_decoded_headers_accumulator_reset_reason_;
   // Visitor of the HttpDecoder.
   std::unique_ptr<HttpDecoderVisitor> http_decoder_visitor_;
   // HttpDecoder for processing raw incoming stream frames.
diff --git a/quic/core/http/quic_spdy_stream_test.cc b/quic/core/http/quic_spdy_stream_test.cc
index 3bad6ef..6ac7fe0 100644
--- a/quic/core/http/quic_spdy_stream_test.cc
+++ b/quic/core/http/quic_spdy_stream_test.cc
@@ -3424,6 +3424,31 @@
   EXPECT_EQ(size - 1, size_with_context);
 }
 
+TEST_P(QuicSpdyStreamTest, HeadersAccumulatorNullptr) {
+  if (!UsesHttp3()) {
+    return;
+  }
+
+  Initialize(kShouldProcessData);
+
+  // Creates QpackDecodedHeadersAccumulator in
+  // `qpack_decoded_headers_accumulator_`.
+  std::string headers = HeadersFrame({std::make_pair("foo", "bar")});
+  stream_->OnStreamFrame(QuicStreamFrame(stream_->id(), false, 0, headers));
+
+  // Resets `qpack_decoded_headers_accumulator_`.
+  stream_->OnHeadersDecoded({}, false);
+
+  // This private method should never be called when
+  // `qpack_decoded_headers_accumulator_` is nullptr.  The number 1 identifies
+  // the site where `qpack_decoded_headers_accumulator_` was last reset.
+  EXPECT_CALL(*connection_, CloseConnection(_, _, _));
+  bool result = true;
+  EXPECT_QUIC_BUG(result = QuicSpdyStreamPeer::OnHeadersFrameEnd(stream_),
+                  "b215142466_OnHeadersFrameEnd.: 1$");
+  EXPECT_FALSE(result);
+}
+
 }  // namespace
 }  // namespace test
 }  // namespace quic
diff --git a/quic/core/qpack/qpack_decoded_headers_accumulator.cc b/quic/core/qpack/qpack_decoded_headers_accumulator.cc
index 4e50e72..934cc5f 100644
--- a/quic/core/qpack/qpack_decoded_headers_accumulator.cc
+++ b/quic/core/qpack/qpack_decoded_headers_accumulator.cc
@@ -7,6 +7,7 @@
 #include "absl/strings/string_view.h"
 #include "quic/core/qpack/qpack_decoder.h"
 #include "quic/core/qpack/qpack_header_table.h"
+#include "quic/platform/api/quic_bug_tracker.h"
 
 namespace quic {
 
@@ -88,6 +89,11 @@
   QUICHE_DCHECK(!error_detected_);
   QUICHE_DCHECK(!headers_decoded_);
 
+  if (!decoder_) {
+    QUIC_BUG(b215142466_EndHeaderBlock);
+    return;
+  }
+
   // Might destroy |this|.
   decoder_->EndHeaderBlock();
 }
diff --git a/quic/test_tools/quic_spdy_stream_peer.cc b/quic/test_tools/quic_spdy_stream_peer.cc
index 6db9a4a..ea21081 100644
--- a/quic/test_tools/quic_spdy_stream_peer.cc
+++ b/quic/test_tools/quic_spdy_stream_peer.cc
@@ -28,5 +28,10 @@
   return stream->use_datagram_contexts_;
 }
 
+// static
+bool QuicSpdyStreamPeer::OnHeadersFrameEnd(QuicSpdyStream* stream) {
+  return stream->OnHeadersFrameEnd();
+}
+
 }  // namespace test
 }  // namespace quic
diff --git a/quic/test_tools/quic_spdy_stream_peer.h b/quic/test_tools/quic_spdy_stream_peer.h
index 8a50426..692cbc3 100644
--- a/quic/test_tools/quic_spdy_stream_peer.h
+++ b/quic/test_tools/quic_spdy_stream_peer.h
@@ -24,6 +24,7 @@
   static const QuicIntervalSet<QuicStreamOffset>& unacked_frame_headers_offsets(
       QuicSpdyStream* stream);
   static bool use_datagram_contexts(QuicSpdyStream* stream);
+  static bool OnHeadersFrameEnd(QuicSpdyStream* stream);
 };
 
 }  // namespace test