diff --git a/tests/LuhnAlgorithmTest.php b/tests/LuhnAlgorithmTest.php
index 2faf68a..4aeb9dd 100644
--- a/tests/LuhnAlgorithmTest.php
+++ b/tests/LuhnAlgorithmTest.php
@@ -58,6 +58,7 @@ public function provideIsValid_success()
             [new Number('410321920', 2), true],
             [new Number('3199723370002', 0), true],
             [new Number('8914800000397416568', 5), true],
+            [new Number('12345', 6), false],
         ];
     }
 
diff --git a/tests/NumberTest.php b/tests/NumberTest.php
index f17d076..b03d7ca 100644
--- a/tests/NumberTest.php
+++ b/tests/NumberTest.php
@@ -53,17 +53,17 @@ public function provideFromString_success()
     /**
      * @dataProvider provideFromString_fail
      */
-    public function testFromString_fail($number, $checkDigit, $expected)
+    public function testFromString_fail($number, $expected)
     {
         $this->expectException($expected);
-        new Number($number, $checkDigit);
+        Number::fromString($number);
     }
 
     public function provideFromString_fail()
     {
         return [
-            ['', 1, \InvalidArgumentException::class],
-            ['xyz ', null, \InvalidArgumentException::class],
+            ['', \InvalidArgumentException::class],
+            ['xyz ', \InvalidArgumentException::class],
         ];
     }