diff --git a/src/timer.hpp b/src/timer.hpp
index 3eed608..a171d84 100644
--- a/src/timer.hpp
+++ b/src/timer.hpp
@@ -10,12 +10,12 @@ namespace sc {
         static uint64_t next_id;
         static std::mutex mutex;
 
-        timer_t _tid;
         uint64_t _id {};
         double _time {};
         bool _repeat {};
         void (*_expired_func)(const timer& self);
-        void* _context {};
+        mutable void* _context {};
+        timer_t _tid;
 
         static void callback(union sigval);