diff --git a/src/Enum.php b/src/Enum.php
index 1ee2573..00355df 100644
--- a/src/Enum.php
+++ b/src/Enum.php
@@ -150,7 +150,7 @@ public static function isValidKey($key)
     {
         $array = static::toArray();
 
-        return isset($array[$key]);
+        return isset($array[$key]) || \array_key_exists($key, $array);
     }
 
     /**
diff --git a/tests/EnumTest.php b/tests/EnumTest.php
index 0e8ea8f..3349bd1 100644
--- a/tests/EnumTest.php
+++ b/tests/EnumTest.php
@@ -190,6 +190,7 @@ public function testIsValidKey()
     {
         $this->assertTrue(EnumFixture::isValidKey('FOO'));
         $this->assertFalse(EnumFixture::isValidKey('BAZ'));
+        $this->assertTrue(EnumFixture::isValidKey('PROBLEMATIC_NULL'));
     }
 
     /**