diff --git a/source/mijin/container/optional.hpp b/source/mijin/container/optional.hpp
index 6d49780..da39933 100644
--- a/source/mijin/container/optional.hpp
+++ b/source/mijin/container/optional.hpp
@@ -157,17 +157,16 @@ public:
     [[nodiscard]] constexpr bool empty() const noexcept { return storage_.empty(); }
     inline void reset() noexcept;
 
-
-    template<typename TCallable, typename TErrorCallable = int>
-    auto then(TCallable&& onSuccess, TErrorCallable&& onError = 0)
+    template<typename TSelf, typename TCallable, typename TErrorCallable = int>
+    auto then(this TSelf&& self, TCallable&& onSuccess, TErrorCallable&& onError = 0)
     {
         using result_t = std::invoke_result_t<TCallable, std::add_rvalue_reference_t<TValue>>;
 
         if constexpr (std::is_same_v<result_t, void>)
         {
-            if (!empty())
+            if (!self.empty())
             {
-                std::invoke(std::forward<TCallable>(onSuccess), get());
+                std::invoke(std::forward<TCallable>(onSuccess), self.get());
             }
             else if constexpr (!std::is_same_v<TErrorCallable, int>)
             {
@@ -176,9 +175,9 @@ public:
         }
         else if constexpr (requires(result_t obj) { {obj == NULL_OPTIONAL} -> std::convertible_to<bool>; })
         {
-            if (!empty())
+            if (!self.empty())
             {
-                return std::invoke(std::forward<TCallable>(onSuccess), get());
+                return std::invoke(std::forward<TCallable>(onSuccess), self.get());
             }
             else
             {