diff --git a/src/inja.hpp b/src/inja.hpp
index a45e54d..c98ded8 100644
--- a/src/inja.hpp
+++ b/src/inja.hpp
@@ -347,7 +347,7 @@ struct Parsed {
 	};
 
 	using Arguments = std::vector<ElementExpression>;
-	using CallbackSignature = std::pair<std::string, int>;
+	using CallbackSignature = std::pair<std::string, size_t>;
 };