diff --git a/libfuse/lib/fuse_lowlevel.c b/libfuse/lib/fuse_lowlevel.c
index 0bdf9371..bc299626 100644
--- a/libfuse/lib/fuse_lowlevel.c
+++ b/libfuse/lib/fuse_lowlevel.c
@@ -281,6 +281,7 @@ fill_open(struct fuse_open_out   *arg_,
     {
       arg_->open_flags |= FOPEN_PASSTHROUGH;
       arg_->backing_id = ffi_->backing_id;
+      fprintf(stderr,"passthrough: %d\n",arg_->backing_id);
     }
 }