diff --git a/Model/File/Writer/YamlWriter.php b/Model/File/Writer/YamlWriter.php
index 15d24cf..c0aad1a 100644
--- a/Model/File/Writer/YamlWriter.php
+++ b/Model/File/Writer/YamlWriter.php
@@ -94,6 +94,10 @@ private function getIndentation($length, $string = ' ')
      */
     private function prepareValue($value)
     {
+        if ($value === null) {
+            return '';
+        }
+
         if (is_numeric($value)) {
             return $value;
         }