diff --git a/src/PolylangTypes.php b/src/PolylangTypes.php
index 0b9b1ab..bd68a7b 100644
--- a/src/PolylangTypes.php
+++ b/src/PolylangTypes.php
@@ -19,7 +19,7 @@ function __action_graphql_register_types()
         $language_codes = [];
 
         foreach (pll_languages_list() as $lang) {
-            $language_codes[strtoupper($lang)] = $lang;
+            $language_codes[str_replace('-', '_', strtoupper($lang))] = $lang;
         }
 
         if (empty($language_codes)) {