diff --git a/coroutine.h b/coroutine.h
index 14a197c..51e212c 100644
--- a/coroutine.h
+++ b/coroutine.h
@@ -1,6 +1,8 @@
 #ifndef COROUTINE_H_
 #define COROUTINE_H_
 
+#include <stddef.h>
+
 // # What is a Coroutine?
 //
 // Coroutine is a lightweight user space thread with its own stack that can