diff --git a/pybind11_protobuf/check_unknown_fields.cc b/pybind11_protobuf/check_unknown_fields.cc
index dca0e25..794234d 100644
--- a/pybind11_protobuf/check_unknown_fields.cc
+++ b/pybind11_protobuf/check_unknown_fields.cc
@@ -62,7 +62,7 @@ bool MessageMayContainExtensionsRecursive(const ::google::protobuf::Descriptor*
 bool MessageMayContainExtensionsMemoized(const ::google::protobuf::Descriptor* descriptor) {
   static auto* memoized = new MayContainExtensionsMap();
   static absl::Mutex lock;
-  absl::MutexLock l(&lock);
+  absl::MutexLock l(lock);
   return MessageMayContainExtensionsRecursive(descriptor, memoized);
 }
 
@@ -187,11 +187,11 @@ absl::optional<std::string> CheckRecursively(
   const auto* root_descriptor = message->GetDescriptor();
   HasUnknownFields search{py_proto_api, root_descriptor};
   if (!search.FindUnknownFieldsRecursive(message, 0u)) {
-    return absl::nullopt;
+    return std::nullopt;
   }
   if (GetAllowList()->count(MakeAllowListKey(root_descriptor->full_name(),
                                              search.FieldFQN())) != 0) {
-    return absl::nullopt;
+    return std::nullopt;
   }
   return search.BuildErrorMessage();
 }