diff --git a/src/string/utf8_len.hpp b/src/string/utf8_len.hpp
index 86cb195..852efa8 100644
--- a/src/string/utf8_len.hpp
+++ b/src/string/utf8_len.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 #include <locale>
 #include <codecvt>