diff --git a/tensorflow_text/core/kernels/phrase_tokenizer.cc b/tensorflow_text/core/kernels/phrase_tokenizer.cc
index 98abddd08..c4bf98a81 100644
--- a/tensorflow_text/core/kernels/phrase_tokenizer.cc
+++ b/tensorflow_text/core/kernels/phrase_tokenizer.cc
@@ -43,13 +43,13 @@ namespace text {
     return absl::InvalidArgumentError(
         "PhraseTokenizerConfig or required fields are null.");
   }
-  tokenizer.trie_ = absl::make_unique<sentencepiece::DoubleArrayTrie>(
+  tokenizer.trie_ = std::make_unique<sentencepiece::DoubleArrayTrie>(
       tokenizer.phrase_config_->vocab_trie()->nodes());
   tokenizer.prob_ = static_cast<float>(tokenizer.phrase_config_->prob()) / 100;
   const auto& ws_config = tokenizer.phrase_config_->whitespace_config();
   tokenizer.whitespace_config_str_ =
       absl::string_view(ws_config->c_str(), ws_config->size());
-  tokenizer.whitespace_tokenizer_ = absl::make_unique<WhitespaceTokenizer>(
+  tokenizer.whitespace_tokenizer_ = std::make_unique<WhitespaceTokenizer>(
       WhitespaceTokenizerConfig(tokenizer.whitespace_config_str_));
   tokenizer.split_end_punctuation_ =
       tokenizer.phrase_config_->split_end_punctuation();
diff --git a/tensorflow_text/core/kernels/string_vocab.cc b/tensorflow_text/core/kernels/string_vocab.cc
index a2c239a93..3cc84594e 100644
--- a/tensorflow_text/core/kernels/string_vocab.cc
+++ b/tensorflow_text/core/kernels/string_vocab.cc
@@ -33,7 +33,7 @@ LookupStatus StringVocab::Contains(absl::string_view key, bool* value) const {
 absl::optional<int> StringVocab::LookupId(absl::string_view key) const {
   auto it = index_map_.find(key);
   if (it == index_map_.end()) {
-    return absl::nullopt;
+    return std::nullopt;
   } else {
     return it->second;
   }
@@ -42,7 +42,7 @@ absl::optional<int> StringVocab::LookupId(absl::string_view key) const {
 // Returns the key of `vocab_id` or empty if `vocab_id` is not valid.
 absl::optional<absl::string_view> StringVocab::LookupWord(int vocab_id) const {
   if (vocab_id >= vocab_.size() || vocab_id < 0) {
-    return absl::nullopt;
+    return std::nullopt;
   }
   return vocab_[vocab_id];
 }