diff --git a/src/Storage.php b/src/Storage.php
index 3028fdd..675d7a4 100644
--- a/src/Storage.php
+++ b/src/Storage.php
@@ -1,7 +1,7 @@
 <?php
 namespace Ely\TempMailBuster;
 
-class Storage
+class Storage implements StorageInterface
 {
     /**
      * @var array of strings, which contains masks for temp mail services
@@ -17,7 +17,7 @@ class Storage
     }
 
     /**
-     * @return array with current items
+     * @inheritdoc
      */
     public function getItems()
     {
@@ -25,20 +25,18 @@ class Storage
     }
 
     /**
-     * @param array $items override current items with passed values
-     * @return static
+     * @inheritdoc
      */
-    public function setItems(array $items)
+    public function setItems($items)
     {
-        $this->items = $items;
+        $this->items = (array)$items;
         return $this;
     }
 
     /**
-     * @param string|array $items item or items, that will be merged to items
-     * @return static
+     * @inheritdoc
      */
-    public function append($items)
+    public function appendItems($items)
     {
         $items = (array)$items;
         $this->items = array_merge($this->items, $items);
diff --git a/src/StorageInterface.php b/src/StorageInterface.php
new file mode 100644
index 0000000..54052f2
--- /dev/null
+++ b/src/StorageInterface.php
@@ -0,0 +1,22 @@
+<?php
+namespace Ely\TempMailBuster;
+
+interface StorageInterface
+{
+    /**
+     * @return array with current items
+     */
+    public function getItems();
+
+    /**
+     * @param array $items replace current items with passed values
+     * @return static
+     */
+    public function setItems($items);
+
+    /**
+     * @param string|array $items item or items, that will be merged to exists array of strings
+     * @return static
+     */
+    public function appendItems($items);
+}
diff --git a/src/TempMailBuster.php b/src/TempMailBuster.php
index d27308f..0d8085e 100644
--- a/src/TempMailBuster.php
+++ b/src/TempMailBuster.php
@@ -4,11 +4,11 @@ namespace Ely\TempMailBuster;
 class TempMailBuster
 {
     /**
-     * @var Storage
+     * @var StorageInterface
      */
     private $primaryStorage;
     /**
-     * @var Storage|null
+     * @var StorageInterface|null
      */
     private $secondaryStorage;
     /**
@@ -17,9 +17,10 @@ class TempMailBuster
     private $isWhitelistMode = false;
 
     /**
-     * @param Storage $storage
+     * @param StorageInterface $primaryStorage
+     * @param StorageInterface|null $secondaryStorage
      */
-    public function __construct(Storage $primaryStorage, Storage $secondaryStorage = null)
+    public function __construct(StorageInterface $primaryStorage, StorageInterface $secondaryStorage = null)
     {
         $this->primaryStorage = $primaryStorage;
         $this->secondaryStorage = $secondaryStorage;
@@ -62,7 +63,7 @@ class TempMailBuster
     }
 
     /**
-     * @return Storage
+     * @return StorageInterface
      */
     public function getPrimaryStorage()
     {
@@ -70,17 +71,17 @@ class TempMailBuster
     }
 
     /**
-     * @param Storage $primaryStorage
+     * @param StorageInterface $primaryStorage
      * @return static
      */
-    public function setPrimaryStorage(Storage $primaryStorage)
+    public function setPrimaryStorage(StorageInterface $primaryStorage)
     {
         $this->primaryStorage = $primaryStorage;
         return $this;
     }
 
     /**
-     * @return Storage|null
+     * @return StorageInterface|null
      */
     public function getSecondaryStorage()
     {
@@ -88,10 +89,10 @@ class TempMailBuster
     }
 
     /**
-     * @param Storage|null $secondaryStorage
+     * @param StorageInterface|null $secondaryStorage
      * @return static
      */
-    public function setSecondaryStorage(Storage $secondaryStorage = null)
+    public function setSecondaryStorage(StorageInterface $secondaryStorage = null)
     {
         $this->secondaryStorage = $secondaryStorage;
         return $this;
diff --git a/tests/StorageTest.php b/tests/StorageTest.php
index eea4466..9f7bf12 100644
--- a/tests/StorageTest.php
+++ b/tests/StorageTest.php
@@ -9,21 +9,21 @@ class StorageTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(['item'], $storage->getItems());
     }
 
-    public function testAppend()
-    {
-        $storage = new Storage(['item1']);
-        $this->assertEquals($storage, $storage->append(['item2']));
-        $this->assertEquals(['item1', 'item2'], $storage->getItems());
-
-        $storage = new Storage(['item1']);
-        $this->assertEquals($storage, $storage->append('item2'));
-        $this->assertEquals(['item1', 'item2'], $storage->getItems());
-    }
-
     public function testSetItems()
     {
         $storage = new Storage(['item1']);
         $this->assertEquals($storage, $storage->setItems(['item2']));
         $this->assertEquals(['item2'], $storage->getItems());
     }
+
+    public function testAppendItems()
+    {
+        $storage = new Storage(['item1']);
+        $this->assertEquals($storage, $storage->appendItems(['item2']));
+        $this->assertEquals(['item1', 'item2'], $storage->getItems());
+
+        $storage = new Storage(['item1']);
+        $this->assertEquals($storage, $storage->appendItems('item2'));
+        $this->assertEquals(['item1', 'item2'], $storage->getItems());
+    }
 }