--- a/src/google/protobuf/json/internal/parser.cc
+++ b/src/google/protobuf/json/internal/parser.cc
@@ -972,13 +972,29 @@
     // Assume approximately six-letter words, so add one extra space for an
     // underscore for every six bytes.
     snake_path.reserve(path.size() * 7 / 6);
+    // Port from protobuf 21.x
+    bool is_quoted = false;
+    bool is_escaping = false;
     for (char c : path) {
+      // Outputs quoted string as-is.
+      if (is_quoted) {
+        snake_path.push_back(c);
+        if (is_escaping) {
+          is_escaping = false;
+        } else if (c == '\\') {
+          is_escaping = true;
+        } else if (c == '\"') {
+          is_quoted = false;
+        }
+        continue;
+      }
       if (absl::ascii_isdigit(c) || absl::ascii_islower(c) || c == '.') {
         snake_path.push_back(c);
       } else if (absl::ascii_isupper(c)) {
         snake_path.push_back('_');
         snake_path.push_back(absl::ascii_tolower(c));
       } else if (lex.options().allow_legacy_syntax) {
+        is_quoted = c == '\"';
         snake_path.push_back(c);
       } else {
         return str->loc.Invalid("unexpected character in FieldMask");
@@ -1314,6 +1330,9 @@
     s = absl::InvalidArgumentError(
         "extraneous characters after end of JSON object");
   }
+  if (s.ok() && !message->IsInitialized()) {
+    s = absl::InvalidArgumentError("Not all fileds are sets");
+  }
 
   PROTOBUF_DLOG(INFO) << "json2/status: " << s;
   PROTOBUF_DLOG(INFO) << "json2/output: " << message->DebugString();