[Yaml] Fix warning

src/Symfony/Component/Yaml/Parser.php

@@ -167,7 +167,7 @@ class Parser
                     $value = Inline::load($this->lines[0]);
                     if (is_array($value)) {
                         $first = reset($value);
-                        if ('*' === substr($first, 0, 1)) {
+                        if (is_string($first) && '*' === substr($first, 0, 1)) {
                             $data = array();
                             foreach ($value as $alias) {
                                 $data[] = $this->refs[substr($alias, 1)];