diff --git a/include/heim_threads.h b/include/heim_threads.h
index a80e801e6..bc01b943f 100644
--- a/include/heim_threads.h
+++ b/include/heim_threads.h
@@ -147,6 +147,8 @@ typedef unsigned long HEIM_PRIV_thread_key;
 
 #elif defined(_WIN32)
 
+#define _Atomic
+
 typedef struct heim_mutex {
     HANDLE	h;
 } heim_mutex_t;