moving nodes in a tree

[Dave Howorth <dhoworth mrc-lmb cam ac uk>]

I'm trying to use $tree_store->move_after ($iter, $position) and
$tree_store->move_before ($iter, $position) and am getting errors like
this:  Gtk-WARNING **: Given children are not in the same level

but I've called $tree_store->iter_depth ($iter) on both arguments
immediately before calling the move methods and I get the same numerical
result for both. i.e.

$model->iter_depth($source_iter) => 4
$model->iter_depth($destination_iter) => 4
$model->move_after($source_iter, $destination_iter)
  => Gtk-WARNING **: Given children are not in the same level

Am I misunderstanding the docs, or is there some other gotcha?

Module version 1.144
Built for gtk+ 2.6.4
Running with gtk+ 2.6.4

Cheers, Dave