diff --git a/src/AppBundle/Service/Packagist/PackagistServiceProvider.php b/src/AppBundle/Service/Packagist/PackagistServiceProvider.php
index 7057121a..0375c0de 100644
--- a/src/AppBundle/Service/Packagist/PackagistServiceProvider.php
+++ b/src/AppBundle/Service/Packagist/PackagistServiceProvider.php
@@ -144,7 +144,7 @@ private function excludeMaintainers(array $maintainers)
     private function calculateChecksum(array $fields = array()) {
         $string = '';
         foreach ($fields as $field) {
-            $string += $field;
+            $string .= $field;
         }
         return md5($string);
     }