diff --git a/src/CryptKey.php b/src/CryptKey.php
index adcacfd3..aedeafb0 100644
--- a/src/CryptKey.php
+++ b/src/CryptKey.php
@@ -51,6 +51,8 @@ class CryptKey
     /**
      * @param string $key
      *
+     * @throws \RuntimeException
+     *
      * @return string
      */
     private function saveKeyToFile($key)
@@ -58,7 +60,9 @@ class CryptKey
         $keyPath = sys_get_temp_dir() . '/' . sha1($key) . '.key';
 
         if (!file_exists($keyPath) && !touch($keyPath)) {
+            // @codeCoverageIgnoreStart
             throw new \RuntimeException('"%s" key file could not be created', $keyPath);
+            // @codeCoverageIgnoreEnd
         }
 
         file_put_contents($keyPath, $key);
diff --git a/tests/CryptKeyTest.php b/tests/CryptKeyTest.php
new file mode 100644
index 00000000..c7f7f4a0
--- /dev/null
+++ b/tests/CryptKeyTest.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace LeagueTests\Utils;
+
+use League\OAuth2\Server\CryptKey;
+
+class CryptKeyTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @expectedException \LogicException
+     */
+    public function testNoFile()
+    {
+        new CryptKey('undefined file');
+    }
+
+    public function testKeyCreation()
+    {
+        $keyFile = __DIR__ . '/Stubs/public.key';
+        $key = new CryptKey($keyFile, 'secret');
+
+        $this->assertEquals('file://' . $keyFile, $key->getKeyPath());
+        $this->assertEquals('secret', $key->getPassPhrase());
+    }
+
+    public function testKeyFileCreation()
+    {
+        $keyContent = file_get_contents(__DIR__ . '/Stubs/public.key');
+        $key = new CryptKey($keyContent);
+
+        $this->assertEquals(
+            'file://' . sys_get_temp_dir() . '/' . sha1($keyContent) . '.key',
+            $key->getKeyPath()
+        );
+    }
+}