diff --git a/include/chaiscript/extras/string_id.hpp b/include/chaiscript/extras/string_id.hpp
index c06c2db..66883b8 100644
--- a/include/chaiscript/extras/string_id.hpp
+++ b/include/chaiscript/extras/string_id.hpp
@@ -1,3 +1,4 @@
+#pragma once
 
 #include <cmath>
 #include <memory>