diff --git a/dlls/d3d8/buffer.c b/dlls/d3d8/buffer.c
index 6ca8829..cd68f6c 100644
--- a/dlls/d3d8/buffer.c
+++ b/dlls/d3d8/buffer.c
@@ -17,6 +17,7 @@
  */
 
 #include "config.h"
+#include <assert.h>
 #include "d3d8_private.h"
 
 WINE_DEFAULT_DEBUG_CHANNEL(d3d8);
@@ -571,3 +572,12 @@
 
     return D3D_OK;
 }
+
+IDirect3DIndexBuffer8Impl *unsafe_impl_from_IDirect3DIndexBuffer8(IDirect3DIndexBuffer8 *iface)
+{
+    if (!iface)
+        return NULL;
+    assert(iface->lpVtbl == &d3d8_indexbuffer_vtbl);
+
+    return impl_from_IDirect3DIndexBuffer8(iface);
+}