diff --git a/composer.json b/composer.json
index 449b4ad..88e94cd 100644
--- a/composer.json
+++ b/composer.json
@@ -8,7 +8,7 @@
         "php-http/message": "^1.0",
         "php-http/client-common": "^1.0",
         "php-http/discovery": "^1.0",
-        "webmozart/assert": "^1.1"
+        "webmozart/assert": "^1.2"
     },
     "require-dev": {
         "phpunit/phpunit": "~4.8",
diff --git a/src/Mailgun/Assert.php b/src/Mailgun/Assert.php
index cb98d3e..99033ab 100644
--- a/src/Mailgun/Assert.php
+++ b/src/Mailgun/Assert.php
@@ -1,5 +1,12 @@
 <?php
 
+/*
+ * Copyright (C) 2013-2016 Mailgun
+ *
+ * This software may be modified and distributed under the terms
+ * of the MIT license.  See the LICENSE file for details.
+ */
+
 namespace Mailgun;
 
 use Mailgun\Exception\InvalidArgumentException;
@@ -11,8 +18,8 @@ use Mailgun\Exception\InvalidArgumentException;
  */
 class Assert extends \Webmozart\Assert\Assert
 {
-    protected static function createInvalidArgumentException($message)
+    protected static function reportInvalidArgument($message)
     {
-        return new InvalidArgumentException($message);
+        throw new InvalidArgumentException($message);
     }
 }
diff --git a/tests/Api/StatsTest.php b/tests/Api/StatsTest.php
index f8b38b5..ef8718a 100644
--- a/tests/Api/StatsTest.php
+++ b/tests/Api/StatsTest.php
@@ -29,15 +29,14 @@ class StatsTest extends TestCase
         $api->total('domain', $data);
     }
 
-    // /**
-    //  * @expectedException \Mailgun\Exception\InvalidArgumentException
-    //  */
-    // public function testTotalInvalidArgument()
-    // {
-    //     $api = $this->getApiMock();
-    //
-    //     $api->total('');
-    // }
+    /**
+     * @expectedException \Mailgun\Exception\InvalidArgumentException
+     */
+    public function testTotalInvalidArgument()
+    {
+        $api = $this->getApiMock();
+        $api->total('');
+    }
 
     public function testAll()
     {
@@ -54,13 +53,13 @@ class StatsTest extends TestCase
         $api->all('domain', $data);
     }
 
-    // /**
-    //  * @expectedException \Mailgun\Exception\InvalidArgumentException
-    //  */
-    // public function testAllInvalidArgument()
-    // {
-    //     $api = $this->getApiMock();
-    //
-    //     $api->all('');
-    // }
+    /**
+     * @expectedException \Mailgun\Exception\InvalidArgumentException
+     */
+    public function testAllInvalidArgument()
+    {
+        $api = $this->getApiMock();
+
+        $api->all('');
+    }
 }