diff --git a/src/PHPHtmlParser/Selector.php b/src/PHPHtmlParser/Selector.php
index e0e1ac73..6910f3d6 100644
--- a/src/PHPHtmlParser/Selector.php
+++ b/src/PHPHtmlParser/Selector.php
@@ -6,6 +6,7 @@
 use PHPHtmlParser\Dom\InnerNode;
 use PHPHtmlParser\Dom\LeafNode;
 use PHPHtmlParser\Exceptions\ChildNotFoundException;
+use Countable;
 
 /**
  * Class Selector
@@ -168,7 +169,9 @@ protected function parseSelectorString($selector)
     protected function seek(array $nodes, array $rule, array $options)
     {
         // XPath index
-        if (count($rule['tag']) > 0 &&
+        if ($rule['tag'] instanceof Countable && 
+            count($rule['tag']) > 0 &&
+            $rule['key'] instanceof Countable &&
             count($rule['key']) > 0 &&
             is_numeric($rule['key'])
         ) {