diff --git a/Classes/PHPExcel/Shared/OLERead.php b/Classes/PHPExcel/Shared/OLERead.php
index f67eeac..0ce44e4 100644
--- a/Classes/PHPExcel/Shared/OLERead.php
+++ b/Classes/PHPExcel/Shared/OLERead.php
@@ -25,7 +25,8 @@
  * @version    ##VERSION##, ##DATE##
  */
 
-define('IDENTIFIER_OLE', pack('CCCCCCCC', 0xd0, 0xcf, 0x11, 0xe0, 0xa1, 0xb1, 0x1a, 0xe1));
+defined('IDENTIFIER_OLE') ||
+    define('IDENTIFIER_OLE', pack('CCCCCCCC', 0xd0, 0xcf, 0x11, 0xe0, 0xa1, 0xb1, 0x1a, 0xe1));
 
 class PHPExcel_Shared_OLERead {
 	private $data = '';