diff --git a/src/Utils.php b/src/Utils.php
index 358e6ae..7dae895 100644
--- a/src/Utils.php
+++ b/src/Utils.php
@@ -25,7 +25,7 @@ class Utils
     public static function assign($obj, array $vars, array $requiredKeys = [])
     {
         foreach ($requiredKeys as $key) {
-            if (!isset($key, $vars)) {
+            if (!isset($vars[$key])) {
                 throw new InvalidArgumentException("Key {$key} is expected to be set and not to be null");
             }
         }
diff --git a/tests/UtilsTest.php b/tests/UtilsTest.php
new file mode 100644
index 0000000..cadb73d
--- /dev/null
+++ b/tests/UtilsTest.php
@@ -0,0 +1,22 @@
+<?php
+namespace GraphQL\Tests;
+
+use GraphQL\Utils;
+
+class UtilsTest extends \PHPUnit_Framework_TestCase
+{
+    public function testAssignThrowsExceptionOnMissingRequiredKey()
+    {
+        $object = new \stdClass();
+        $object->requiredKey = 'value';
+
+        try {
+            Utils::assign($object, [], ['requiredKey']);
+            $this->fail('Expected exception not thrown');
+        } catch (\InvalidArgumentException $e) {
+            $this->assertEquals(
+                "Key requiredKey is expected to be set and not to be null",
+                $e->getMessage());
+        }
+    }
+}