diff --git a/reactos/dll/win32/rpcrt4/cproxy.c b/reactos/dll/win32/rpcrt4/cproxy.c
index 084285a5458..26e5c416c9c 100644
--- a/reactos/dll/win32/rpcrt4/cproxy.c
+++ b/reactos/dll/win32/rpcrt4/cproxy.c
@@ -56,7 +56,7 @@ typedef struct {
 
 static const IRpcProxyBufferVtbl StdProxy_Vtbl;
 
-#define ICOM_THIS_MULTI(impl,field,iface) impl* const This=(impl*)((char*)(iface) - offsetof(impl,field))
+#define ICOM_THIS_MULTI(impl,field,iface) impl* const This=(impl*)((char*)(iface) - FIELD_OFFSET(impl,field))
 
 /* How the Windows stubless proxy thunks work is explained at
  * http://msdn.microsoft.com/library/en-us/dnmsj99/html/com0199.asp,