diff --git a/multi_timer.h b/multi_timer.h
index 55cf42c..edc3e38 100644
--- a/multi_timer.h
+++ b/multi_timer.h
@@ -7,6 +7,7 @@
 #define _MULTI_TIMER_H_
 
 #include "stdint.h"
+#include "stddef.h"
 
 typedef struct Timer {
     uint32_t timeout;