diff --git a/src/Tests/ArrayKeyVisitor.php b/src/Tests/ArrayKeyVisitor.php
index 5001aed..9e8e696 100644
--- a/src/Tests/ArrayKeyVisitor.php
+++ b/src/Tests/ArrayKeyVisitor.php
@@ -42,7 +42,7 @@ public function enterNode(Node $node)
 			foreach ($node->items as $item)
 			{
 				/** @var ArrayItem $item */
-				if ($item->key instanceof String_)
+				if ($item instanceof ArrayItem && $item->key instanceof String_)
 				{
 					$this->keys[] = $item->key->value;
 				}