Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 15646)
+++ wp-includes/user.php	(working copy)
@@ -24,6 +24,7 @@
  */
 function wp_signon( $credentials = '', $secure_cookie = '' ) {
 	if ( empty($credentials) ) {
+		$credentials = array();
 		if ( ! empty($_POST['log']) )
 			$credentials['user_login'] = $_POST['log'];
 		if ( ! empty($_POST['pwd']) )
@@ -37,6 +38,9 @@
 	else
 		$credentials['remember'] = false;
 
+	// set default credential values
+	$credentials = array_merge( array('user_login' => '', 'user_password' => ''),  $credentials);
+
 	// TODO do we deprecate the wp_authentication action?
 	do_action_ref_array('wp_authenticate', array(&$credentials['user_login'], &$credentials['user_password']));