Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 23182)
+++ wp-includes/user.php	(working copy)
@@ -1452,6 +1452,8 @@
 
 	// First, get all of the original fields
 	$user_obj = get_userdata( $ID );
+	if ( ! $user_obj )
+		return new WP_Error( 'invalid_user_id', __( 'Invalid user ID.' ) );
 
 	$user = $user_obj->to_array();