diff --git a/transport_mbox.c b/transport_mbox.c
index 852c0a4..3d71820 100644
--- a/transport_mbox.c
+++ b/transport_mbox.c
@@ -264,7 +264,7 @@
 
 	MSG_DBG("LPC address of current window: 0x%.8x\n", lpc_addr);
 
-	return lpc_addr >> context->block_size_shift;
+	return lpc_addr >> context->backend.block_size_shift;
 }
 
 static int mbox_handle_create_window(struct mbox_context *context, bool ro,