diff --git a/src/PhpWord/Style/Language.php b/src/PhpWord/Style/Language.php
index 2ccc44c7..7b2de51e 100644
--- a/src/PhpWord/Style/Language.php
+++ b/src/PhpWord/Style/Language.php
@@ -228,7 +228,9 @@ final class Language extends AbstractStyle
      */
     private function validateLocale($locale)
     {
-        $locale = str_replace('_', '-', $locale);
+        if ($locale !== null) {
+            $locale = str_replace('_', '-', $locale);
+        }
 
         if (strlen($locale) === 2) {
             return strtolower($locale) . '-' . strtoupper($locale);