diff --git a/src/Lepo.i18n/LocalizationBuilderExtensions.cs b/src/Lepo.i18n/LocalizationBuilderExtensions.cs
index f3b5977..b5c104f 100644
--- a/src/Lepo.i18n/LocalizationBuilderExtensions.cs
+++ b/src/Lepo.i18n/LocalizationBuilderExtensions.cs
@@ -3,6 +3,8 @@
 // Copyright (C) Leszek Pomianowski and Lepo.i18n Contributors.
 // All Rights Reserved.
 
+using System.Threading;
+
 namespace Lepo.i18n;
 
 /// <summary>