diff --git a/src/Symfony/Extractor/FailOverExtractor.php b/src/Symfony/Extractor/FailOverExtractor.php
index 7c2b4a1..62f0bda 100644
--- a/src/Symfony/Extractor/FailOverExtractor.php
+++ b/src/Symfony/Extractor/FailOverExtractor.php
@@ -42,7 +42,7 @@ public function getTypes(string $class, string $property, array $context = []):
     {
         $cacheKey = \sha1($class . $property);
 
-        if (isset($this->localStorage[$cacheKey]) === false)
+        if (\array_key_exists($cacheKey, $this->localStorage) === false)
         {
             $this->localStorage[$cacheKey] = null;