diff --git a/C4/Koha.pm b/C4/Koha.pm
index 1b6160a76e..ec65be9691 100644
--- a/C4/Koha.pm
+++ b/C4/Koha.pm
@@ -1680,6 +1680,7 @@ sub NormalizeISBN {
         elsif ( $format eq 'ISBN-13' ) {
             $isbn = $isbn->as_isbn13();
         }
+        return unless $isbn;
 
         if ($strip_hyphens) {
             $string = $isbn->as_string( [] );