diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000000..24d97f2b22
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,3 @@
+BasedOnStyle: Google
+DerivePointerAlignment: false
+PointerAlignment: Left
diff --git a/.github/workflows/bazel_tests.yml b/.github/workflows/bazel_tests.yml
index 062342aeea..b6e9fe24dd 100644
--- a/.github/workflows/bazel_tests.yml
+++ b/.github/workflows/bazel_tests.yml
@@ -1,4 +1,4 @@
-name: Bazel Tests
+name: Check ClangFormat
 
 on:
   push:
@@ -10,27 +10,11 @@ on:
   workflow_dispatch:
 
 jobs:
-
-  ubuntu:
-    runs-on: ${{ matrix.os }}
-
-    strategy:
-      fail-fast: false   # Don't cancel all jobs if one fails.
-      matrix:
-        include:
-          - { CC: clang, os: ubuntu-20.04, flags: "" }
-          - { CC: clang, os: ubuntu-20.04, flags: "-c opt" }  # Some warnings only fire with -c opt
-          - { CC: gcc, os: ubuntu-20.04, flags: "-c opt" }
-          - { CC: clang, os: ubuntu-20.04, flags: "--//:fasttable_enabled=true -- -cmake:test_generated_files" }
-          - { CC: clang, os: ubuntu-20.04, flags: "--config=asan -- -benchmarks:benchmark -python/..." }
-          - { CC: clang, os: macos-11, flags: "" }
-
+  check_clang_format:
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
-      - name: Setup Python venv
-        run: rm -rf /tmp/venv && python3 -m venv /tmp/venv
-      - name: Install dependencies
-        run: sudo apt install -y ${{ matrix.install }}
-        if: matrix.install != ''
-      - name: Run tests
-        run: cd ${{ github.workspace }} && PATH=/tmp/venv/bin:$PATH CC=${{ matrix.CC }} bazel test --test_output=errors ... ${{ matrix.flags }}
+      - name: Run ClangFormat
+        run: find . | grep -E '\.(c|h|cc)$' | grep -E -v '^./(third_party|cmake)' | xargs clang-format -i
+      - name: Check for differences
+        run: git diff --exit-code
diff --git a/benchmarks/benchmark.cc b/benchmarks/benchmark.cc
index 06c16fd91a..368780e1f3 100644
--- a/benchmarks/benchmark.cc
+++ b/benchmarks/benchmark.cc
@@ -33,6 +33,7 @@
 #include "benchmarks/descriptor_sv.pb.h"
 #include "google/ads/googleads/v7/services/google_ads_service.upbdefs.h"
 #include "google/protobuf/descriptor.pb.h"
+#include "google/protobuf/dynamic_message.h"
 #include "upb/def.hpp"
 
 upb_StringView descriptor = benchmarks_descriptor_proto_upbdefinit.descriptor;
@@ -70,47 +71,89 @@ static void BM_ArenaInitialBlockOneAlloc(benchmark::State& state) {
 }
 BENCHMARK(BM_ArenaInitialBlockOneAlloc);
 
-static void BM_LoadDescriptor_Upb(benchmark::State& state) {
-  size_t bytes_per_iter = 0;
-  for (auto _ : state) {
-    upb::SymbolTable symtab;
-    upb_benchmark_DescriptorProto_getmsgdef(symtab.ptr());
-    bytes_per_iter = _upb_DefPool_BytesLoaded(symtab.ptr());
+enum LoadDescriptorMode {
+  NoLayout,
+  WithLayout,
+};
+
+// This function is mostly copied from upb/def.c, but it is modified to avoid
+// passing in the pre-generated mini-tables, in order to force upb to compute
+// them dynamically.  Generally you would never want to do this, but we want to
+// simulate the cost we would pay if we were loading these types purely from
+// descriptors, with no mini-tales available.
+bool LoadDefInit_BuildLayout(upb_DefPool* s, const _upb_DefPool_Init* init,
+                             size_t* bytes) {
+  _upb_DefPool_Init** deps = init->deps;
+  google_protobuf_FileDescriptorProto* file;
+  upb_Arena* arena;
+  upb_Status status;
+
+  upb_Status_Clear(&status);
+
+  if (upb_DefPool_FindFileByName(s, init->filename)) {
+    return true;
   }
-  state.SetBytesProcessed(state.iterations() * bytes_per_iter);
+
+  arena = upb_Arena_New();
+
+  for (; *deps; deps++) {
+    if (!LoadDefInit_BuildLayout(s, *deps, bytes)) goto err;
+  }
+
+  file = google_protobuf_FileDescriptorProto_parse_ex(
+      init->descriptor.data, init->descriptor.size, NULL,
+      kUpb_DecodeOption_AliasString, arena);
+  *bytes += init->descriptor.size;
+
+  if (!file) {
+    upb_Status_SetErrorFormat(
+        &status,
+        "Failed to parse compiled-in descriptor for file '%s'. This should "
+        "never happen.",
+        init->filename);
+    goto err;
+  }
+
+  // KEY DIFFERENCE: Here we pass in only the descriptor, and not the
+  // pre-generated minitables.
+  if (!upb_DefPool_AddFile(s, file, &status)) {
+    goto err;
+  }
+
+  upb_Arena_Free(arena);
+  return true;
+
+err:
+  fprintf(stderr,
+          "Error loading compiled-in descriptor for file '%s' (this should "
+          "never happen): %s\n",
+          init->filename, upb_Status_ErrorMessage(&status));
+  exit(1);
 }
-BENCHMARK(BM_LoadDescriptor_Upb);
 
+template <LoadDescriptorMode Mode>
 static void BM_LoadAdsDescriptor_Upb(benchmark::State& state) {
   size_t bytes_per_iter = 0;
   for (auto _ : state) {
     upb::SymbolTable symtab;
-    google_ads_googleads_v7_services_SearchGoogleAdsRequest_getmsgdef(
-        symtab.ptr());
-    bytes_per_iter = _upb_DefPool_BytesLoaded(symtab.ptr());
-  }
-  state.SetBytesProcessed(state.iterations() * bytes_per_iter);
-}
-BENCHMARK(BM_LoadAdsDescriptor_Upb);
-
-static void BM_LoadDescriptor_Proto2(benchmark::State& state) {
-  for (auto _ : state) {
-    protobuf::Arena arena;
-    protobuf::StringPiece input(descriptor.data, descriptor.size);
-    auto proto =
-        protobuf::Arena::CreateMessage<protobuf::FileDescriptorProto>(&arena);
-    protobuf::DescriptorPool pool;
-    bool ok = proto->ParseFrom<protobuf::MessageLite::kMergePartial>(input) &&
-              pool.BuildFile(*proto) != nullptr;
-    if (!ok) {
-      printf("Failed to add file.\n");
-      exit(1);
+    if (Mode == NoLayout) {
+      google_ads_googleads_v7_services_SearchGoogleAdsRequest_getmsgdef(
+          symtab.ptr());
+      bytes_per_iter = _upb_DefPool_BytesLoaded(symtab.ptr());
+    } else {
+      bytes_per_iter = 0;
+      LoadDefInit_BuildLayout(
+          symtab.ptr(),
+          &google_ads_googleads_v7_services_google_ads_service_proto_upbdefinit,
+          &bytes_per_iter);
     }
   }
-  state.SetBytesProcessed(state.iterations() * descriptor.size);
+  state.SetBytesProcessed(state.iterations() * bytes_per_iter);
 }
-BENCHMARK(BM_LoadDescriptor_Proto2);
+BENCHMARK_TEMPLATE(BM_LoadAdsDescriptor_Upb, NoLayout);
+BENCHMARK_TEMPLATE(BM_LoadAdsDescriptor_Upb, WithLayout);
 
+template <LoadDescriptorMode Mode>
 static void BM_LoadAdsDescriptor_Proto2(benchmark::State& state) {
   extern _upb_DefPool_Init
       google_ads_googleads_v7_services_google_ads_service_proto_upbdefinit;
@@ -136,10 +179,22 @@ static void BM_LoadAdsDescriptor_Proto2(benchmark::State& state) {
       }
       bytes_per_iter += input.size();
     }
+
+    if (Mode == WithLayout) {
+      protobuf::DynamicMessageFactory factory;
+      const protobuf::Descriptor* d = pool.FindMessageTypeByName(
+          "google.ads.googleads.v7.services.SearchGoogleAdsResponse");
+      if (!d) {
+        printf("Failed to find descriptor.\n");
+        exit(1);
+      }
+      factory.GetPrototype(d);
+    }
   }
   state.SetBytesProcessed(state.iterations() * bytes_per_iter);
 }
-BENCHMARK(BM_LoadAdsDescriptor_Proto2);
+BENCHMARK_TEMPLATE(BM_LoadAdsDescriptor_Proto2, NoLayout);
+BENCHMARK_TEMPLATE(BM_LoadAdsDescriptor_Proto2, WithLayout);
 
 enum CopyStrings {
   Copy,
@@ -154,7 +209,6 @@ enum ArenaMode {
 
 template <ArenaMode AMode, CopyStrings Copy>
 static void BM_Parse_Upb_FileDesc(benchmark::State& state) {
-  size_t bytes = 0;
   for (auto _ : state) {
     upb_Arena* arena;
     if (AMode == InitBlock) {
@@ -170,7 +224,6 @@ static void BM_Parse_Upb_FileDesc(benchmark::State& state) {
       printf("Failed to parse.\n");
       exit(1);
     }
-    bytes += descriptor.size;
     upb_Arena_Free(arena);
   }
   state.SetBytesProcessed(state.iterations() * descriptor.size);
@@ -223,7 +276,6 @@ using FileDescSV = ::upb_benchmark::sv::FileDescriptorProto;
 
 template <class P, ArenaMode AMode, CopyStrings kCopy>
 void BM_Parse_Proto2(benchmark::State& state) {
-  size_t bytes = 0;
   constexpr protobuf::MessageLite::ParseFlags kParseFlags =
       kCopy == Copy
           ? protobuf::MessageLite::ParseFlags::kMergePartial
@@ -237,7 +289,6 @@ void BM_Parse_Proto2(benchmark::State& state) {
       printf("Failed to parse.\n");
       exit(1);
     }
-    bytes += descriptor.size;
   }
   state.SetBytesProcessed(state.iterations() * descriptor.size);
 }
@@ -247,12 +298,10 @@ BENCHMARK_TEMPLATE(BM_Parse_Proto2, FileDesc, InitBlock, Copy);
 BENCHMARK_TEMPLATE(BM_Parse_Proto2, FileDescSV, InitBlock, Alias);
 
 static void BM_SerializeDescriptor_Proto2(benchmark::State& state) {
-  size_t bytes = 0;
   upb_benchmark::FileDescriptorProto proto;
   proto.ParseFromArray(descriptor.data, descriptor.size);
   for (auto _ : state) {
     proto.SerializePartialToArray(buf, sizeof(buf));
-    bytes += descriptor.size;
   }
   state.SetBytesProcessed(state.iterations() * descriptor.size);
 }
diff --git a/tests/BUILD b/tests/BUILD
index f00360711d..3d2474450a 100644
--- a/tests/BUILD
+++ b/tests/BUILD
@@ -83,6 +83,7 @@ proto_library(
     srcs = [
         "test_cpp.proto",
     ],
+    deps = ["@com_google_protobuf//:timestamp_proto"]
 )
 
 upb_proto_library(
diff --git a/tests/bindings/lua/test.proto b/tests/bindings/lua/test.proto
index 4c3b64edb5..c751436016 100644
--- a/tests/bindings/lua/test.proto
+++ b/tests/bindings/lua/test.proto
@@ -32,3 +32,38 @@ message TestLargeFieldNumber {
 message TestTimestamp {
   optional google.protobuf.Timestamp ts = 1;
 }
+
+message HelloRequest {
+  optional uint32 id = 1;
+  optional uint32 random_name_a0 = 2;
+  optional uint32 random_name_a1 = 3;
+  optional uint32 random_name_a2 = 4;
+  optional uint32 random_name_a3 = 5;
+  optional uint32 random_name_a4 = 6;
+  optional uint32 random_name_a5 = 7;
+  optional uint32 random_name_a6 = 8;
+  optional uint32 random_name_a7 = 9;
+  optional uint32 random_name_a8 = 10;
+  optional uint32 random_name_a9 = 11;
+  optional uint32 random_name_b0 = 12;
+  optional uint32 random_name_b1 = 13;
+  optional uint32 random_name_b2 = 14;
+  optional uint32 random_name_b3 = 15;
+  optional uint32 random_name_b4 = 16;
+  optional uint32 random_name_b5 = 17;
+  optional uint32 random_name_b6 = 18;
+  optional uint32 random_name_b7 = 19;
+  optional uint32 random_name_b8 = 20;
+  optional uint32 random_name_b9 = 21;
+  optional uint32 random_name_c0 = 22;
+  optional uint32 random_name_c1 = 23;
+  optional uint32 random_name_c2 = 24;
+  optional uint32 random_name_c3 = 25;
+  optional uint32 random_name_c4 = 26;
+  optional uint32 random_name_c5 = 27;
+  optional uint32 random_name_c6 = 28;
+  optional uint32 random_name_c7 = 29;
+  optional uint32 random_name_c8 = 30;
+  optional uint32 random_name_c9 = 31;
+  optional string version = 32;
+}
diff --git a/tests/bindings/lua/test_upb.lua b/tests/bindings/lua/test_upb.lua
index 5f1aae8837..329c78ac6b 100644
--- a/tests/bindings/lua/test_upb.lua
+++ b/tests/bindings/lua/test_upb.lua
@@ -831,6 +831,14 @@ function test_gc()
   end
 end
 
+function test_b9440()
+  local m = upb_test.HelloRequest()
+  m.id = 8
+  assert_equal(8, m.id)
+  m.version = "1"
+  assert_equal(8, m.id)
+end
+
 local stats = lunit.main()
 
 if stats.failed > 0 or stats.errors > 0 then
diff --git a/tests/test.proto b/tests/test.proto
index 9c24aa05ab..f4b94b4032 100644
--- a/tests/test.proto
+++ b/tests/test.proto
@@ -11,3 +11,38 @@ message MessageName {
   optional int32 field1 = 1;
   optional int32 field2 = 2;
 }
+
+message HelloRequest {
+  optional uint32 id = 1;
+  optional uint32 random_name_a0 = 2;
+  optional uint32 random_name_a1 = 3;
+  optional uint32 random_name_a2 = 4;
+  optional uint32 random_name_a3 = 5;
+  optional uint32 random_name_a4 = 6;
+  optional uint32 random_name_a5 = 7;
+  optional uint32 random_name_a6 = 8;
+  optional uint32 random_name_a7 = 9;
+  optional uint32 random_name_a8 = 10;
+  optional uint32 random_name_a9 = 11;
+  optional uint32 random_name_b0 = 12;
+  optional uint32 random_name_b1 = 13;
+  optional uint32 random_name_b2 = 14;
+  optional uint32 random_name_b3 = 15;
+  optional uint32 random_name_b4 = 16;
+  optional uint32 random_name_b5 = 17;
+  optional uint32 random_name_b6 = 18;
+  optional uint32 random_name_b7 = 19;
+  optional uint32 random_name_b8 = 20;
+  optional uint32 random_name_b9 = 21;
+  optional uint32 random_name_c0 = 22;
+  optional uint32 random_name_c1 = 23;
+  optional uint32 random_name_c2 = 24;
+  optional uint32 random_name_c3 = 25;
+  optional uint32 random_name_c4 = 26;
+  optional uint32 random_name_c5 = 27;
+  optional uint32 random_name_c6 = 28;
+  optional uint32 random_name_c7 = 29;
+  optional uint32 random_name_c8 = 30;
+  optional uint32 random_name_c9 = 31;
+  optional string version = 32;
+}
diff --git a/tests/test_cpp.cc b/tests/test_cpp.cc
index 6424452c4f..cbc4b08aa0 100644
--- a/tests/test_cpp.cc
+++ b/tests/test_cpp.cc
@@ -35,6 +35,8 @@
 #include <set>
 #include <sstream>
 
+#include "google/protobuf/timestamp.upb.h"
+#include "google/protobuf/timestamp.upbdefs.h"
 #include "gtest/gtest.h"
 #include "tests/test_cpp.upb.h"
 #include "tests/test_cpp.upbdefs.h"
@@ -148,3 +150,37 @@ TEST(Cpp, JsonNull) {
   EXPECT_EQ(0, strcmp(str_f.default_value().str_val.data, "abc"));
   EXPECT_EQ(3, str_f.default_value().str_val.size);
 }
+
+TEST(Cpp, TimestampEncoder) {
+  upb::SymbolTable symtab;
+  upb::Arena arena;
+  upb::MessageDefPtr md(google_protobuf_Timestamp_getmsgdef(symtab.ptr()));
+  google_protobuf_Timestamp* timestamp_upb =
+      google_protobuf_Timestamp_new(arena.ptr());
+  google_protobuf_Timestamp* timestamp_upb_decoded =
+      google_protobuf_Timestamp_new(arena.ptr());
+
+  long timestamps[] = {
+      253402300799,  // 9999-12-31T23:59:59Z
+      1641006000,    // 2022-01-01T03:00:00Z
+      0,             // 1970-01-01T00:00:00Z
+      -31525200,     // 1969-01-01T03:00:00Z
+      -2208988800,   // 1900-01-01T00:00:00Z
+      -62135596800,  // 0000-01-01T00:00:00Z
+  };
+
+  for (long timestamp : timestamps) {
+    google_protobuf_Timestamp_set_seconds(timestamp_upb, timestamp);
+
+    char json[128];
+    size_t size = upb_JsonEncode(timestamp_upb, md.ptr(), NULL, 0, json,
+                                 sizeof(json), NULL);
+    bool result = upb_JsonDecode(json, size, timestamp_upb_decoded, md.ptr(),
+                                 NULL, 0, arena.ptr(), NULL);
+    const long timestamp_decoded =
+        google_protobuf_Timestamp_seconds(timestamp_upb_decoded);
+
+    ASSERT_TRUE(result);
+    EXPECT_EQ(timestamp, timestamp_decoded);
+  }
+}
diff --git a/tests/test_generated_code.cc b/tests/test_generated_code.cc
index 89372890f9..4f3d715e0b 100644
--- a/tests/test_generated_code.cc
+++ b/tests/test_generated_code.cc
@@ -30,9 +30,10 @@
  * upb/def.c and tests/conformance_upb.c, respectively).
  */
 
+#include "gtest/gtest.h"
 #include "src/google/protobuf/test_messages_proto3.upb.h"
 #include "tests/test.upb.h"
-#include "gtest/gtest.h"
+#include "upb/upb.hpp"
 
 #define MIN(x, y) ((x) < (y) ? (x) : (y))
 
@@ -295,12 +296,14 @@ static void check_int32_map_one_entry(
   EXPECT_EQ(
       1, protobuf_test_messages_proto3_TestAllTypesProto3_map_int32_int32_size(
              msg));
-  EXPECT_TRUE(protobuf_test_messages_proto3_TestAllTypesProto3_map_int32_int32_get(
-      msg, test_int32, &val));
+  EXPECT_TRUE(
+      protobuf_test_messages_proto3_TestAllTypesProto3_map_int32_int32_get(
+          msg, test_int32, &val));
   EXPECT_EQ(val, test_int32_2);
 
-  EXPECT_FALSE(protobuf_test_messages_proto3_TestAllTypesProto3_map_int32_int32_get(
-      msg, test_int32_3, &val));
+  EXPECT_FALSE(
+      protobuf_test_messages_proto3_TestAllTypesProto3_map_int32_int32_get(
+          msg, test_int32_3, &val));
 
   /* Test that iteration reveals a single k/v pair in the map. */
   iter = kUpb_Map_Begin;
@@ -411,6 +414,16 @@ TEST(GeneratedCode, TestRepeated) {
   upb_Arena_Free(arena);
 }
 
+TEST(GeneratedCode, Issue9440) {
+  upb::Arena arena;
+  upb_test_HelloRequest* msg = upb_test_HelloRequest_new(arena.ptr());
+  upb_test_HelloRequest_set_id(msg, 8);
+  EXPECT_EQ(8, upb_test_HelloRequest_id(msg));
+  char str[] = "1";
+  upb_test_HelloRequest_set_version(msg, upb_StringView{str, strlen(str)});
+  EXPECT_EQ(8, upb_test_HelloRequest_id(msg));
+}
+
 TEST(GeneratedCode, NullDecodeBuffer) {
   upb_Arena* arena = upb_Arena_New();
   protobuf_test_messages_proto3_TestAllTypesProto3* msg =
diff --git a/tests/test_table.cc b/tests/test_table.cc
index 5ba2276438..e1a1d40733 100644
--- a/tests/test_table.cc
+++ b/tests/test_table.cc
@@ -460,7 +460,7 @@ TEST_P(IntTableTest, TestIntTable) {
   uint32_t largest_key = 0;
   std::map<uint32_t, uint32_t> m;
   std::unordered_map<uint32_t, uint32_t> hm;
-  for (const auto& key: keys_) {
+  for (const auto& key : keys_) {
     largest_key = UPB_MAX((int32_t)largest_key, key);
     table.Insert(key, key * 2);
     m[key] = key * 2;
diff --git a/upb/def.c b/upb/def.c
index d8db187c7e..9fe8c7e9fa 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -1574,7 +1574,7 @@ static void make_layout(symtab_addctx* ctx, const upb_MessageDef* m) {
   }
 
   /* Account for space used by hasbits. */
-  l->size = div_round_up(hasbit, 8);
+  l->size = hasbit ? div_round_up(hasbit + 1, 8) : 0;
 
   /* Allocate non-oneof fields. */
   for (int i = 0; i < m->field_count; i++) {
diff --git a/upb/encode.c b/upb/encode.c
index d800df84b5..f09dd4a245 100644
--- a/upb/encode.c
+++ b/upb/encode.c
@@ -544,8 +544,8 @@ static void encode_message(upb_encstate* e, const upb_Message* msg,
      * other. */
     size_t ext_count;
     const upb_Message_Extension* ext = _upb_Message_Getexts(msg, &ext_count);
-    const upb_Message_Extension* end = ext + ext_count;
     if (ext_count) {
+      const upb_Message_Extension* end = ext + ext_count;
       for (; ext != end; ext++) {
         if (UPB_UNLIKELY(m->ext == upb_ExtMode_IsMessageSet)) {
           encode_msgset_item(e, ext);
diff --git a/upb/json_encode.c b/upb/json_encode.c
index 64826e34dd..e0b35f1320 100644
--- a/upb/json_encode.c
+++ b/upb/json_encode.c
@@ -162,7 +162,8 @@ static void jsonenc_timestamp(jsonenc* e, const upb_Message* msg,
    * Fliegel, H. F., and Van Flandern, T. C., "A Machine Algorithm for
    *   Processing Calendar Dates," Communications of the Association of
    *   Computing Machines, vol. 11 (1968), p. 657.  */
-  L = (int)(seconds / 86400) + 68569 + 2440588;
+  seconds += 62135596800;  // Ensure seconds is positive.
+  L = (int)(seconds / 86400) - 719162 + 68569 + 2440588;
   N = 4 * L / 146097;
   L = L - (146097 * N + 3) / 4;
   I = 4000 * (L + 1) / 1461001;
diff --git a/upb/upb.c b/upb/upb.c
index a702d48e55..55730bbe82 100644
--- a/upb/upb.c
+++ b/upb/upb.c
@@ -341,7 +341,6 @@ static void upb_FixLocale(char* p) {
   }
 }
 
-
 void _upb_EncodeRoundTripDouble(double val, char* buf, size_t size) {
   assert(size >= kUpb_RoundTripBufferSize);
   snprintf(buf, size, "%.*g", DBL_DIG, val);
diff --git a/upb/upb_internal.h b/upb/upb_internal.h
index d14a175836..1eb166f7d3 100644
--- a/upb/upb_internal.h
+++ b/upb/upb_internal.h
@@ -61,9 +61,7 @@ struct upb_Arena {
 // the beginning.
 //
 // The given buffer size must be at least kUpb_RoundTripBufferSize.
-enum {
-  kUpb_RoundTripBufferSize = 32
-};
+enum { kUpb_RoundTripBufferSize = 32 };
 void _upb_EncodeRoundTripDouble(double val, char* buf, size_t size);
 void _upb_EncodeRoundTripFloat(float val, char* buf, size_t size);
 
diff --git a/upbc/message_layout.cc b/upbc/message_layout.cc
index dd2edcbc0b..be63a105c8 100644
--- a/upbc/message_layout.cc
+++ b/upbc/message_layout.cc
@@ -190,7 +190,7 @@ void MessageLayout::PlaceNonOneofFields(
   }
 
   // Place hasbits at the beginning.
-  hasbit_bytes_ = DivRoundUp(hasbit_count_, 8);
+  hasbit_bytes_ = hasbit_count_ ? DivRoundUp(hasbit_count_ + 1, 8) : 0;
   Place(SizeAndAlign{{hasbit_bytes_, hasbit_bytes_}, {1, 1}});
 
   // Place non-oneof fields.