diff --git a/src/mcpack2pb/parser.cpp b/src/mcpack2pb/parser.cpp
index 5c785dc45f..92d7a22021 100644
--- a/src/mcpack2pb/parser.cpp
+++ b/src/mcpack2pb/parser.cpp
@@ -577,6 +577,15 @@ double UnparsedValue::as_double(const char* var) {
 }
 
 void UnparsedValue::as_string(std::string* out, const char* var) {
+    if (_size < 1) {
+        // A string field must contain at least the trailing '\0'.
+        // Reject _size == 0 here, otherwise `_size - 1' underflows and
+        // resize() throws an uncaught exception. Clear `out' so callers
+        // that reuse the string do not keep a stale value.
+        out->clear();
+        _stream->set_bad();
+        return;
+    }
     out->resize(_size - 1);
     if (_stream->cutn(&(*out)[0], _size - 1) != _size - 1) {
         CHECK(false) << "Not enough data for " << var;
diff --git a/test/brpc_mcpack2pb_unittest.cpp b/test/brpc_mcpack2pb_unittest.cpp
index 68de0522fe..6718cae93b 100644
--- a/test/brpc_mcpack2pb_unittest.cpp
+++ b/test/brpc_mcpack2pb_unittest.cpp
@@ -23,6 +23,86 @@
 
 namespace {
 
+TEST(Mcpack2pbParserTest, StringFieldWithZeroValueSize) {
+    // A 51-byte mcpack2 frame whose `service_name' string field has
+    // value_size == 0. This used to underflow in UnparsedValue::as_string()
+    // (resize(_size - 1) with _size == 0) and throw std::length_error, which
+    // is not caught on the request path and therefore crashes the server.
+    const unsigned char data[] = {
+        0x10, 0x00, 0x2d, 0x00, 0x00, 0x00,
+        0x01, 0x00, 0x00, 0x00,
+        0xa0, 0x08, 0x1e, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x00,
+        0x01, 0x00, 0x00, 0x00,
+        0x10, 0x00, 0x14, 0x00, 0x00, 0x00,
+        0x01, 0x00, 0x00, 0x00,
+        0xd0, 0x0d, 0x00, 0x73, 0x65, 0x72, 0x76, 0x69, 0x63, 0x65,
+        0x5f, 0x6e, 0x61, 0x6d, 0x65, 0x00,
+    };
+    butil::IOBuf body;
+    body.append(data, sizeof(data));
+
+    butil::IOBufAsZeroCopyInputStream zc_stream(body);
+    mcpack2pb::InputStream stream(&zc_stream);
+    ASSERT_NE(0u, mcpack2pb::unbox(&stream));
+
+    mcpack2pb::ObjectIterator it1(&stream, body.size() - stream.popped_bytes());
+    bool found_content = false;
+    for (; it1 != NULL; ++it1) {
+        if (it1->name == "content") {
+            found_content = true;
+            break;
+        }
+    }
+    ASSERT_TRUE(found_content);
+    ASSERT_EQ(mcpack2pb::FIELD_ARRAY, it1->value.type());
+
+    mcpack2pb::ArrayIterator it2(it1->value);
+    ASSERT_TRUE(it2 != NULL);
+    bool found_service_name = false;
+    for (mcpack2pb::ObjectIterator it3(*it2); it3 != NULL; ++it3) {
+        if (it3->name == "service_name") {
+            found_service_name = true;
+            ASSERT_EQ(mcpack2pb::FIELD_STRING, it3->value.type());
+            std::string service_name = "stale";
+            it3->value.as_string(&service_name, "service_name");
+            // A zero-sized string field must be rejected gracefully instead of
+            // throwing (resize(SIZE_MAX)) and crashing the process.
+            EXPECT_FALSE(it3->value.stream()->good());
+            // The output string must be cleared so callers that reuse the
+            // string do not keep a stale value.
+            EXPECT_TRUE(service_name.empty());
+            break;
+        }
+    }
+    ASSERT_TRUE(found_service_name);
+}
+
+TEST(Mcpack2pbParserTest, ParseStringField) {
+    // A valid object {"msg":"abc"}.
+    const unsigned char data[] = {
+        0x10, 0x00, 0x0f, 0x00, 0x00, 0x00,
+        0x01, 0x00, 0x00, 0x00,
+        0xd0, 0x04, 0x04,
+        0x6d, 0x73, 0x67, 0x00,
+        0x61, 0x62, 0x63, 0x00,
+    };
+    butil::IOBuf body;
+    body.append(data, sizeof(data));
+
+    butil::IOBufAsZeroCopyInputStream zc_stream(body);
+    mcpack2pb::InputStream stream(&zc_stream);
+    ASSERT_NE(0u, mcpack2pb::unbox(&stream));
+
+    mcpack2pb::ObjectIterator it(&stream, body.size() - stream.popped_bytes());
+    ASSERT_TRUE(it != NULL);
+    EXPECT_EQ("msg", it->name.as_string());
+    ASSERT_EQ(mcpack2pb::FIELD_STRING, it->value.type());
+    std::string value;
+    it->value.as_string(&value, "msg");
+    EXPECT_EQ("abc", value);
+    EXPECT_TRUE(stream.good());
+}
+
 TEST(Mcpack2pbParserTest, ArrayItemCountIsCappedToRemainingBytes) {
     // An mcpack array whose header claims item_count = 0x7fffffff (INT32_MAX)
     // but contains no actual items. The raw item_count is fed by the