diff --git a/File.class.php b/File.class.php
index e44028b..25ebabe 100644
--- a/File.class.php
+++ b/File.class.php
@@ -12,8 +12,8 @@ abstract class File
     public static function getInstance($data)
     {
         $instance = new static($data);
-        if (is_null($data)) {
-            throw new ErrorException('Empty data for Image::getInstance().');
+        if (is_null($data) || $data == '') {
+            throw new ErrorException('Empty data for File::getInstance().');
         }
         if (!is_array($data)) {
             $data = json_decode($data, true);
@@ -22,7 +22,7 @@ abstract class File
             }
         }
         foreach ($data as $attribute_name => $attribute_value) {
-            if (property_exists(__CLASS__, $attribute_name)) {
+            if (property_exists($instance, $attribute_name)) {
                 $instance->{$attribute_name} = $attribute_value;
             }
         }
diff --git a/Image.class.php b/Image.class.php
index b4093d5..194639b 100644
--- a/Image.class.php
+++ b/Image.class.php
@@ -14,7 +14,8 @@ class Image extends File
     const SIZE_200_160 = '200x160';
     const SIZE_300_240 = '300x240';
     const SIZE_550_440 = '550x440';
-    const SIZE_SKI = '1000x';
+    const SIZE_SKI = 'x';
+    const SIZE_INSTRUCTION = 'x';
 
     public static $sizes = array(
         self::SIZE_100_80,