Index: src/libcfa/concurrency/monitor
===================================================================
--- src/libcfa/concurrency/monitor	(revision cb0e6de65962fd19c1eae698acaec94f1a41b56c)
+++ src/libcfa/concurrency/monitor	(revision 72dc82a4d117f5fcafe19599a7b13602476df4b4)
@@ -18,4 +18,6 @@
 #define MONITOR_H
 
+#include <stddef.h>
+
 #include "assert"
 #include "invoke.h"
@@ -23,5 +25,6 @@
 
 static inline void ?{}(monitor_desc * this) {
-	this->owner = 0;
+	this->owner = NULL;
+      this->stack_owner = NULL;
 	this->recursion = 0;
 }