diff --git a/source/mijin/types/script_value.hpp b/source/mijin/types/script_value.hpp
index 03674f3..b78463d 100644
--- a/source/mijin/types/script_value.hpp
+++ b/source/mijin/types/script_value.hpp
@@ -233,7 +233,7 @@ public:
         }
         else if constexpr (std::is_integral_v<type_t>)
         {
-            return toInt().then([](script_int_t val) { return static_cast<type_t>(val); });
+            return toInt().then([](script_int_t val) -> Optional<std::decay_t<T>> { return static_cast<type_t>(val); });
         }
         else if constexpr (std::is_same_v<type_t, script_float_t>)
         {
@@ -241,7 +241,7 @@ public:
         }
         else if constexpr (std::is_floating_point_v<type_t>)
         {
-            return toFloat().then([](script_float_t val) { return static_cast<type_t>(val); });
+            return toFloat().then([](script_float_t val) -> Optional<std::decay_t<T>> { return static_cast<type_t>(val); });
         }
         else if constexpr (std::is_same_v<type_t, std::string>)
         {