diff --git a/composer.json b/composer.json
index 7d384d1..b9be08b 100644
--- a/composer.json
+++ b/composer.json
@@ -3,7 +3,6 @@
 	"type": "library",
 	"description": "Implementation of the Luhn algorithm in PHP. Used in validation of credit card numbers and some national identification numbers.",
 	"prefer-stable": true,
-	"prefer-lowest": true,
 	"minimum-stability": "stable",
 	"keywords": [
 		"luhn",
diff --git a/src/ArgumentIsNotNumericException.php b/src/ArgumentIsNotNumericException.php
new file mode 100644
index 0000000..e79d21a
--- /dev/null
+++ b/src/ArgumentIsNotNumericException.php
@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+declare(strict_types=1);
+
+namespace Nekman\LuhnAlgorithm;
+
+class ArgumentIsNotNumericException extends \InvalidArgumentException
+{
+    /**
+     * ArgumentIsNotNumericException constructor.
+     * @param string $number
+     */
+    public function __construct(string $number)
+    {
+        parent::__construct("Expects \$number to be a number, \"{$number}\" given.", 0, null);
+    }
+}
diff --git a/src/LuhnAlgorithm.php b/src/LuhnAlgorithm.php
index c75a6b9..444d57a 100644
--- a/src/LuhnAlgorithm.php
+++ b/src/LuhnAlgorithm.php
@@ -55,7 +55,7 @@ public function isValid(NumberInterface $number): bool
     public function calcCheckDigit(NumberInterface $number): int
     {
         $checksum = $this->calcChecksum($number);
-        
+
         // Get the last digit of the checksum.
         $checkDigit = $checksum % 10;
 
diff --git a/src/Number.php b/src/Number.php
index a343e33..4228862 100644
--- a/src/Number.php
+++ b/src/Number.php
@@ -52,7 +52,7 @@ class Number implements NumberInterface
     public function __construct(string $number, int $checkDigit = null)
     {
         if (!is_numeric($number)) {
-            throw new \InvalidArgumentException("Expects \$number to be a number, \"{$number}\" given.");
+            throw new ArgumentIsNotNumericException($number);
         }
 
         $this->number = $number;
@@ -72,14 +72,16 @@ public static function fromString(string $input): self
         $input = preg_replace('/[^\d]/', '', $input);
 
         if (!is_numeric($input)) {
-            throw new \InvalidArgumentException("Expects \$input to be a number, \"{$input}\" given.");
+            throw new ArgumentIsNotNumericException($input);
         }
 
+        $lastIndex = strlen($input) - 1;
+
         // Get the last digit.
-        $checkDigit = (int) $input[strlen($input) - 1];
+        $checkDigit = (int) $input[$lastIndex];
 
         // Remove the last digit.
-        $number = substr($input, 0, strlen($input) - 1);
+        $number = substr($input, 0, $lastIndex);
 
         return new self($number, $checkDigit);
     }
@@ -100,11 +102,12 @@ public function getCheckDigit()
         return $this->checkDigit;
     }
 
+    
     /**
      * {@inheritdoc}
      */
     public function __toString()
     {
-        return (string) $this->number . (string) $this->checkDigit;
+        return $this->number . $this->checkDigit;
     }
 }