diff --git a/src/IBAN/Core/IBAN.php b/src/IBAN/Core/IBAN.php
index <HASH>..<HASH> 100644
--- a/src/IBAN/Core/IBAN.php
+++ b/src/IBAN/Core/IBAN.php
@@ -70,6 +70,7 @@ class IBAN
     }
 
     private function getNumericRepresentation($letterRepresentation) {
+        $numericRepresentation = '';
         foreach (str_split($letterRepresentation) as $char) {
             if (array_search($char, \IBAN\Core\Constants::$letterMapping)) {
                 $numericRepresentation .= array_search($char, \IBAN\Core\Constants::$letterMapping) + 9;
Initialized property numericRepresentation