diff --git a/system_scheduler.hpp b/system_scheduler.hpp
index 1bd8a11..9c4fb05 100644
--- a/system_scheduler.hpp
+++ b/system_scheduler.hpp
@@ -1,4 +1,5 @@
 #pragma once
+#include <concepts>
 #include <functional>
 #include <vector>
 
@@ -7,9 +8,9 @@ namespace zecsy
     class system_scheduler final
     {
     public:
-        void add_system(float freq, auto&& func);
+        void add_system(float freq, std::invocable<float> auto&& func);
 
-        void add_system(int freq, auto&& func);
+        void add_system(int freq, std::invocable<float> auto&& func);
 
         void update(float dt);
 
@@ -24,13 +25,15 @@ namespace zecsy
         std::vector<system_handler> systems;
     };
 
-    inline void system_scheduler::add_system(float freq, auto&& func)
+    inline void system_scheduler::add_system(float freq,
+                                             std::invocable<float> auto&& func)
     {
         systems.emplace_back(1.0f / freq, 0.0f,
                              std::forward<decltype(func)>(func));
     }
 
-    inline void system_scheduler::add_system(int freq, auto&& func)
+    inline void system_scheduler::add_system(int freq,
+                                             std::invocable<float> auto&& func)
     {
         add_system(float(freq), func);
     }