diff --git a/src/google/protobuf/extension_set_inl.h b/src/google/protobuf/extension_set_inl.h
index 4554d95083..4fdc3991f4 100644
--- a/src/google/protobuf/extension_set_inl.h
+++ b/src/google/protobuf/extension_set_inl.h
@@ -215,7 +215,8 @@ const char* ExtensionSet::ParseMessageSetItemTmpl(
     if (tag == WireFormatLite::kMessageSetTypeIdTag) {
       uint64_t tmp;
       ptr = ParseBigVarint(ptr, &tmp);
-      GOOGLE_PROTOBUF_PARSER_ASSERT(ptr);
+      // We should fail parsing if type id is 0.
+      GOOGLE_PROTOBUF_PARSER_ASSERT(ptr != nullptr && tmp != 0);
       if (state == State::kNoTag) {
         type_id = tmp;
         state = State::kHasType;
diff --git a/src/google/protobuf/wire_format.cc b/src/google/protobuf/wire_format.cc
index 0e7fedfd33..5359f4e434 100644
--- a/src/google/protobuf/wire_format.cc
+++ b/src/google/protobuf/wire_format.cc
@@ -671,7 +671,8 @@ struct WireFormat::MessageSetParser {
       if (tag == WireFormatLite::kMessageSetTypeIdTag) {
         uint64_t tmp;
         ptr = ParseBigVarint(ptr, &tmp);
-        GOOGLE_PROTOBUF_PARSER_ASSERT(ptr);
+        // We should fail parsing if type id is 0.
+        GOOGLE_PROTOBUF_PARSER_ASSERT(ptr != nullptr && tmp != 0);
         if (state == State::kNoTag) {
           type_id = tmp;
           state = State::kHasType;
diff --git a/src/google/protobuf/wire_format_lite.h b/src/google/protobuf/wire_format_lite.h
index efe48ce161..3097a14f51 100644
--- a/src/google/protobuf/wire_format_lite.h
+++ b/src/google/protobuf/wire_format_lite.h
@@ -1890,7 +1890,8 @@ bool ParseMessageSetItemImpl(io::CodedInputStream* input, MS ms) {
     switch (tag) {
       case WireFormatLite::kMessageSetTypeIdTag: {
         uint32_t type_id;
-        if (!input->ReadVarint32(&type_id)) return false;
+        // We should fail parsing if type id is 0.
+        if (!input->ReadVarint32(&type_id) || type_id == 0) return false;
         if (state == State::kNoTag) {
           last_type_id = type_id;
           state = State::kHasType;
diff --git a/src/google/protobuf/wire_format_unittest.inc b/src/google/protobuf/wire_format_unittest.inc
index 4c7d047c71..45a74bc964 100644
--- a/src/google/protobuf/wire_format_unittest.inc
+++ b/src/google/protobuf/wire_format_unittest.inc
@@ -579,6 +579,31 @@ TEST(WireFormatTest, ParseMessageSet) {
   EXPECT_EQ(message_set.DebugString(), dynamic_message_set.DebugString());
 }
 
+TEST(WireFormatTest, MessageSetUnknownButValidTypeId) {
+  const char encoded[] = {
+      013,     // 1: SGROUP
+      032, 2,  // 3:LEN 2
+      010, 0,  // 1:0
+      020, 4,  // 2:4
+      014      // 1: EGROUP
+  };
+  PROTO2_WIREFORMAT_UNITTEST::TestMessageSet message;
+  EXPECT_TRUE(message.ParseFromArray(encoded, sizeof(encoded)));
+}
+
+TEST(WireFormatTest, MessageSetInvalidTypeId) {
+  // "type_id" is 0 and should fail to parse.
+  const char encoded[] = {
+      013,     // 1: SGROUP
+      032, 2,  // 3:LEN 2
+      010, 0,  // 1:0
+      020, 0,  // 2:0
+      014      // 1: EGROUP
+  };
+  PROTO2_WIREFORMAT_UNITTEST::TestMessageSet message;
+  EXPECT_FALSE(message.ParseFromArray(encoded, sizeof(encoded)));
+}
+
 namespace {
 std::string BuildMessageSetItemStart() {
   std::string data;