diff --git a/Documentation/conf.py b/Documentation/conf.py
index f6823cf01275aa..7fadb3b8329343 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -135,7 +135,7 @@
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False
 
-primary_domain = 'C'
+primary_domain = 'c'
 highlight_language = 'none'
 
 # -- Options for HTML output ----------------------------------------------