calling _revisioning_delete_revision gives fatal error

When using _revisioning_delete_revision from revisioning_api.inc, Drupal gives the following error:
Recoverable fatal error: Object of class stdClass could not be converted to string in _revisioning_delete_revision() (line 414 of /Applications/MAMP/htdocs/sites/all/modules/revisioning/revisioning_api.inc).

The culprit is on ln 414:
module_invoke_all('node_' . $node_revision, 'delete revision');

$node_revision is a Node object, not a string.