diff --git a/rostests/winetests/ole32/compobj.c b/rostests/winetests/ole32/compobj.c
index 9d469b4b174..755d15a9f66 100644
--- a/rostests/winetests/ole32/compobj.c
+++ b/rostests/winetests/ole32/compobj.c
@@ -35,6 +35,8 @@
 
 #include "wine/test.h"
 
+extern const IID GUID_NULL;
+
 /* functions that are not present on all versions of Windows */
 HRESULT (WINAPI * pCoInitializeEx)(LPVOID lpReserved, DWORD dwCoInit);
 HRESULT (WINAPI * pCoGetObjectContext)(REFIID riid, LPVOID *ppv);