Make Parent Terms as Single

I would like to make the multiple selection in parent releations to single.. so i add
$form['relations']['parent'] = array(
'#type' => 'hierarchical_select',
'#title' => t('Parents'),
'#required' => FALSE,
'#default_value' => $parent,
'#config' => $config,
'#multiple' => FALSE,

'#multiple' => FALSE, is not working in hs_taxonomy.module but it was executed in core taxonomy.admin.inc... but i need to use heirarhical select.. so could anyone try to solve this issue???