diff --git a/lib/Kolab/FreeBusy/Source.php b/lib/Kolab/FreeBusy/Source.php
index 5b0213f..56a77f1 100644
--- a/lib/Kolab/FreeBusy/Source.php
+++ b/lib/Kolab/FreeBusy/Source.php
@@ -1,123 +1,124 @@
 <?php
 
 /**
  * This file is part of the Kolab Server Free/Busy Service
  *
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  *
  * Copyright (C) 2013, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 namespace Kolab\FreeBusy;
 
 /**
  * Abstract class to fetch free/busy data from a specific source
  */
 abstract class Source
 {
 	protected $config = array();
 	protected $cached = false;
 
 	/**
 	 * Factory method creating an instace of Source according to config
 	 *
 	 * @param string Source URI
 	 * @param array Hash array with config
 	 */
 	public static function factory($url, $conf)
 	{
 		$config = parse_url($url);
 		$config['url'] = $url;
 		switch ($config['scheme']) {
 			case 'file':	return new SourceFile($config + $conf);
 			case 'tls':
 			case 'imap':
 			case 'imaps':	return new SourceIMAP($config + $conf);
 			case 'http':
 			case 'https':	return new SourceURL($config + $conf);
 			case 'fbd':
 			case 'fbdaemon': return new SourceFBDaemon($config + $conf);
 			case 'aggregate': return new SourceAggregator($config + $conf);
 		}
 
 		Logger::get('source')->addError("Invalid source configuration: " . $url);
 		return null;
 	}
 
 	/**
 	 * Default constructor
 	 */
 	public function __construct($config)
 	{
 		$this->config = $config;
 	}
 
 	/**
 	 * Retrieve free/busy data for the given user
 	 *
 	 * @param array Hash array with user attributes
 	 */
 	abstract public function getFreeBusyData($user, $extended);
 
 	/**
 	 * Replace all %varname strings in config with values from $user
 	 */
 	protected function getUserConfig($user)
 	{
 		$config = array();
 		foreach ($this->config as $k => $val) {
 			if (is_string($val) && strpos($val, '%') !== false) {
 				$val = preg_replace_callback(
 					'/%\{?([a-z0-9]+)\}?/',
 					function($m) use ($k, $user) {
-						$enc = $k == 'url' || $k == 'query' || $k == 'fbsource';
-						return $enc ? urlencode($user[$m[1]]) : $user[$m[1]];
+						$encode = $k == 'url' || $k == 'query' || $k == 'fbsource';
+						$v = $user[$m[1]] ?? '';
+						return $encode ? urlencode($v) : $v;
 					},
 					$val);
 			}
 
 			$config[$k] = $val;
 		}
 
 		return $config;
 	}
 
 	/**
 	 * Helper method to check if a cached file exists and is still valid
 	 *
 	 * @param array Hash array with (replaced) config properties
 	 * @return string Cached free-busy data or false if cache file doesn't exist or is expired
 	 */
 	protected function getCached($config)
 	{
 		if ($config['cacheto'] && file_exists($config['cacheto'])) {
 			if (empty($config['expires']) || filemtime($config['cacheto']) + Utils::getOffsetSec($config['expires']) >= time()) {
 				$this->cached = true;
 				return file_get_contents($config['cacheto']);
 			}
 		}
 
 		return false;
 	}
 
 	/**
 	 * Return the value of the 'cached' flag
 	 */
 	public function isCached()
 	{
 		return $this->cached;
 	}
 }