diff --git a/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultRequestResponseSerializer.cs b/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultRequestResponseSerializer.cs
index 7669ee7dd1..915c0552dc 100644
--- a/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultRequestResponseSerializer.cs
+++ b/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultRequestResponseSerializer.cs
@@ -24,6 +24,10 @@ internal sealed class DefaultRequestResponseSerializer : SystemTextJsonSerialize
 {
 	private readonly IElasticsearchClientSettings _settings;
 
+#if !NET8_0_OR_GREATER
+	private readonly object _lock = new();
+#endif
+
 	public DefaultRequestResponseSerializer(IElasticsearchClientSettings settings) :
 		base(new DefaultRequestResponseSerializerOptionsProvider(settings))
 	{
@@ -62,15 +66,23 @@ private void LinkSettings(IElasticsearchClientSettings settings)
 		var options = GetJsonSerializerOptions(SerializationFormatting.None);
 		var indentedOptions = GetJsonSerializerOptions(SerializationFormatting.Indented);
 
-		if (!ElasticsearchClient.SettingsTable.TryGetValue(options, out _))
-		{
-			ElasticsearchClient.SettingsTable.Add(options, settings);
-		}
-
-		if (!ElasticsearchClient.SettingsTable.TryGetValue(indentedOptions, out _))
+#if NET8_0_OR_GREATER
+		ElasticsearchClient.SettingsTable.TryAdd(options, settings);
+		ElasticsearchClient.SettingsTable.TryAdd(indentedOptions, settings);
+#else
+		lock (_lock)
 		{
-			ElasticsearchClient.SettingsTable.Add(indentedOptions, settings);
+			if (!ElasticsearchClient.SettingsTable.TryGetValue(options, out _))
+			{
+				ElasticsearchClient.SettingsTable.Add(options, settings);
+			}
+
+			if (!ElasticsearchClient.SettingsTable.TryGetValue(indentedOptions, out _))
+			{
+				ElasticsearchClient.SettingsTable.Add(indentedOptions, settings);
+			}
 		}
+#endif
 	}
 }
 
diff --git a/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultSourceSerializer.cs b/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultSourceSerializer.cs
index de2bc568f4..2b986d4fa0 100644
--- a/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultSourceSerializer.cs
+++ b/src/Elastic.Clients.Elasticsearch/_Shared/Serialization/DefaultSourceSerializer.cs
@@ -21,6 +21,10 @@ namespace Elastic.Clients.Elasticsearch.Serialization;
 public class DefaultSourceSerializer :
 	SystemTextJsonSerializer
 {
+#if !NET8_0_OR_GREATER
+	private readonly object _lock = new();
+#endif
+
 	/// <summary>
 	/// Constructs a new <see cref="DefaultSourceSerializer"/> instance that accepts an <see cref="Action{T}"/> that can
 	/// be provided to customize the default <see cref="JsonSerializerOptions"/>.
@@ -43,15 +47,23 @@ private void LinkSettings(IElasticsearchClientSettings settings)
 		var options = GetJsonSerializerOptions(SerializationFormatting.None);
 		var indentedOptions = GetJsonSerializerOptions(SerializationFormatting.Indented);
 
-		if (!ElasticsearchClient.SettingsTable.TryGetValue(options, out _))
+#if NET8_0_OR_GREATER
+		ElasticsearchClient.SettingsTable.TryAdd(options, settings);
+		ElasticsearchClient.SettingsTable.TryAdd(indentedOptions, settings);
+#else
+		lock (_lock)
 		{
-			ElasticsearchClient.SettingsTable.Add(options, settings);
-		}
+			if (!ElasticsearchClient.SettingsTable.TryGetValue(options, out _))
+			{
+				ElasticsearchClient.SettingsTable.Add(options, settings);
+			}
 
-		if (!ElasticsearchClient.SettingsTable.TryGetValue(indentedOptions, out _))
-		{
-			ElasticsearchClient.SettingsTable.Add(indentedOptions, settings);
+			if (!ElasticsearchClient.SettingsTable.TryGetValue(indentedOptions, out _))
+			{
+				ElasticsearchClient.SettingsTable.Add(indentedOptions, settings);
+			}
 		}
+#endif
 	}
 }