Unable to call drush command using drush_invoke_process()

I have a custom drush command, and as part of that command I'm attempting to call another drush command from another module. In particular, I'm attempting to call the node-export-export command (D6). Based on what I read here, I'm calling drush_invoke_process() as follows:

$export_file_path = '/path/to/file/node_export_nodes.txt';
drush_invoke_process('@self', 'node-export-export', $nids, array('file' => $export_file_path));

where $nids is an array of nid values to export. However, when I run my command, it just hangs. I've stepped through the drush code, and it seems to die at the call to _drush_backend_proc_open() in drush_backend_invoke().

The only thing I could see that might cause a problem is that the command line command that is generated includes

--uri=http://Sites

or

--uri=http://default

where in my case I'm running it at the moment on a local http://mysite domain using a vhost config. Other than that, I'm not sure what I'm doing wrong.

Any suggestions would be appreciated.

Thanks.