diff --git a/src/Scrypt.php b/src/Scrypt.php
index 5e723f7..8d16774 100644
--- a/src/Scrypt.php
+++ b/src/Scrypt.php
@@ -24,7 +24,7 @@ public function __construct($builder)
     {
         parent::__construct('SCRYPT', 0, 8, $builder);
 
-        if (!is_null($builder->getKey()) || strlen($builder->getKey()) === 0) {
+        if (is_null($builder->getKey()) || strlen($builder->getKey()) === 0) {
             throw new \InvalidArgumentException('A non-empty key is required for Scrypt');
         }