diff --git a/source/mijin/async/signal.hpp b/source/mijin/async/signal.hpp
index 5be5196..33459c5 100644
--- a/source/mijin/async/signal.hpp
+++ b/source/mijin/async/signal.hpp
@@ -22,6 +22,9 @@ namespace mijin
 // public constants
 //
 
+using signal_token_t = std::uint32_t;
+inline constexpr signal_token_t INVALID_SIGNAL_TOKEN = std::numeric_limits<signal_token_t>::max();
+
 //
 // public types
 //
@@ -33,7 +36,7 @@ class Signal
 {
 public:
     using handler_t = std::function<void(TArgs...)>;
-    using token_t = std::uint32_t;
+    using token_t = signal_token_t;
 private:
     struct RegisteredHandler
     {