Index: /trunk/src/VBox/Devices/Samples/DrvStorageFilter.cpp
===================================================================
--- /trunk/src/VBox/Devices/Samples/DrvStorageFilter.cpp	(revision 52546)
+++ /trunk/src/VBox/Devices/Samples/DrvStorageFilter.cpp	(revision 52547)
@@ -1,9 +1,9 @@
 /* $Id$ */
 /** @file
- * VBox Sample Driver.
- */
-
-/*
- * Copyright (C) 2012 Oracle Corporation
+ * VBox storage filter driver sample.
+ */
+
+/*
+ * Copyright (C) 2012-2014 Oracle Corporation
  *
  * This file is part of VirtualBox Open Source Edition (OSE), as
@@ -340,4 +340,9 @@
 
     AssertLogRelReturn(pThis->pIMediaBelow, VERR_PDM_MISSING_INTERFACE_BELOW);
+
+    if (!pThis->pIMediaBelow->pfnDiscard)
+        pThis->IMedia.pfnDiscard = NULL;
+    if (!pThis->pIMediaAsyncBelow || !pThis->pIMediaAsyncBelow->pfnStartDiscard)
+        pThis->IMediaAsync.pfnStartDiscard = NULL;
 
     return VINF_SUCCESS;