diff --git a/src/spdx2/agent/spdx2utils.php b/src/spdx2/agent/spdx2utils.php
index 1a096e8d..d41c608a 100644
--- a/src/spdx2/agent/spdx2utils.php
+++ b/src/spdx2/agent/spdx2utils.php
@@ -1,125 +1,126 @@
 <?php
 /*
  * Copyright (C) 2016, Siemens AG
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
  * version 2 as published by the Free Software Foundation.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License along
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  */
 namespace Fossology\SpdxTwo;
 
 class SpdxTwoUtils
 {
   static public $prefix = "LicenseRef-";
 
   /**
    * @param string[] $args
    * @param string $key1
    * @param string $key2
    *
    * @return string[] $args
    */
   static public function preWorkOnArgsFlp($args,$key1,$key2)
   {
     $needle = ' --'.$key2.'=';
     if (is_array($args) &&
         array_key_exists($key1, $args) &&
         strpos($args[$key1],$needle) !== false) {
       $exploded = explode($needle,$args[$key1]);
       $args[$key1] = trim($exploded[0]);
       $args[$key2] = trim($exploded[1]);
     }
     return $args;
   }
 
   /**
    * @param string $license
    * @param callable $spdxValidityChecker
    *
    * @return string
    */
   static public function addPrefixOnDemand($license, $spdxValidityChecker = null)
   {
     if(strpos($license, " OR ") !== false)
     {
       return "(" . $license . ")";
     }
     else
     {
-      if(is_callable($spdxValidityChecker) &&
-          call_user_func($spdxValidityChecker, $license))
+      if($spdxValidityChecker === null ||
+          (is_callable($spdxValidityChecker) &&
+              call_user_func($spdxValidityChecker, $license)))
       {
         return $license;
       }
       else
       {
         return self::$prefix . $license;
       }
     }
   }
 
   static public function addPrefixOnDemandKeys($licenses, $spdxValidityChecker = null)
   {
     $ret = array();
     foreach($licenses as $license=>$text)
     {
       $ret[self::addPrefixOnDemand($license, $spdxValidityChecker)] = $text;
     }
     return $ret;
   }
 
   static public function addPrefixOnDemandList($licenses, $spdxValidityChecker = null)
   {
     $ret = array();
     foreach($licenses as $license)
     {
       $ret[] = self::addPrefixOnDemand($license, $spdxValidityChecker);
     }
     return $ret;
   }
 
   /**
    * @param string[] $licenses
    * @param callable $spdxValidityChecker
    *
    * @return string
    */
   static public function implodeLicenses($licenses, $spdxValidityChecker = null)
   {
     if(!$licenses || !is_array($licenses) || sizeof($licenses) == 0)
     {
       return "";
     }
 
     $licenses = array_map(function ($license) use ($spdxValidityChecker)
     {
       return SpdxTwoUtils::addPrefixOnDemand($license, $spdxValidityChecker);
     },$licenses);
     sort($licenses, SORT_NATURAL | SORT_FLAG_CASE);
 
     if(count($licenses) == 3 &&
        ($index = array_search("Dual-license",$licenses)) !== false)
     {
       return $licenses[$index===0?1:0] . " OR " . $licenses[$index===2?1:2];
     }
     elseif(count($licenses) == 3 &&
         ($index = array_search(self::$prefix . "Dual-license",$licenses)) !== false)
     {
       return $licenses[$index===0?1:0] . " OR " . $licenses[$index===2?1:2];
     }
     else
     {
       // Add prefixes where needed, enclose statments containing ' OR ' with parantheses
       return implode(" AND ", $licenses);
     }
   }
 }