diff --git a/lib/SimpleSAML/Utilities.php b/lib/SimpleSAML/Utilities.php
index 18925686a0..f272a135d5 100644
--- a/lib/SimpleSAML/Utilities.php
+++ b/lib/SimpleSAML/Utilities.php
@@ -310,13 +310,12 @@ public static function addURLparameter($url, $parameter) {
 	 * allowed by configuration.
 	 */
 	public static function checkURLAllowed($url, array $trustedSites = NULL) {
-		$url = self::normalizeURL($url);
-
-		// verify that the URL points to an http or https site
-		if (!preg_match('@^https?://@i', $url)) {
-			throw new SimpleSAML_Error_Exception('Invalid URL: '.$url);
+		if (empty($url)) {
+			return '';
 		}
 
+		$url = self::normalizeURL($url);
+
 		// get the white list of domains
 		if ($trustedSites === NULL) {
 			$trustedSites = SimpleSAML_Configuration::getInstance()->getArray('trusted.url.domains', NULL);