diff --git a/docs/schema/theme.json b/docs/schema/theme.json
index 51b4cbd29..ca168700c 100644
--- a/docs/schema/theme.json
+++ b/docs/schema/theme.json
@@ -45,6 +45,12 @@
       "title": "Site language",
       "markdownDescription": "https://squidfunk.github.io/mkdocs-material/setup/changing-the-language/",
       "oneOf": [
+        {
+          "title": "Site language: Custom",
+          "enum": [
+            "custom"
+          ]
+        },
         {
           "title": "Site language: Afrikaans",
           "enum": [