Updating an existing contact and setting prefix to an empty string results in error at CRM/Utils/Array.php on line 54

Updating an existing contact and setting prefix to an empty string results in the following error:

Warning: array_key_exists(): The first argument should be either a string or an integer in /home/tom/workspace/example/sites/example.com/modules/civicrm/CRM/Utils/Array.php on line 54

The following script is a test that I ran from my civicrm bin directory as a test (thanks dlobo):

<?php

require_once '../../../civicrm.settings.php';
require_once '../CRM/Core/Config.php';
require_once '../CRM/Core/Error.php';
$config = CRM_Core_Config::singleton( );

require_once 'api/v2/Contact.php';
$params = array(
'contact_id' => '17911',
'contact_type' => 'Individual',
'external_identifier' => '58534',
'prefix' => '',
'first_name' => 'Tom KP',
'last_name' => 'Mansfield',
'gender_id' => '',
'organization_name' => '',
'addressee_custom' => 'Tom',
);
$result = civicrm_contact_add( $params );
CRM_Core_Error::debug( $result );
?>

The contact does appear to update successfully.
Commenting out the line 'prefix' => '' makes the error go away.