The first rule of a template can not start with a "/", even if it has a default value

framework/web/UrlRule.php

@@ -125,7 +125,7 @@ class UrlRule extends Object
 				if (isset($this->defaults[$name])) {
 					$length = strlen($match[0][0]);
 					$offset = $match[0][1];
-					if ($this->pattern[$offset - 1] === '/' && $this->pattern[$offset + $length] === '/') {
+					if ($offset > 1 && $this->pattern[$offset - 1] === '/' && $this->pattern[$offset + $length] === '/') {
 						$tr["/<$name>"] = "(/(?P<$name>$pattern))?";
 					} else {
 						$tr["<$name>"] = "(?P<$name>$pattern)?";