diff --git a/src/PDFLib.php b/src/PDFLib.php
index f3cae5d..a5f3d3e 100644
--- a/src/PDFLib.php
+++ b/src/PDFLib.php
@@ -34,6 +34,7 @@ class PDFLib{
     private $imageDeviceCommand;
     private $imageExtention;
     private $pngDownScaleFactor;
+    private $file_prefix;
 
     private $is_os_win = null;
     private $gs_command = null;