diff --git a/sphinx/conf.py b/sphinx/conf.py
index 8bf2b5378..57f564e4d 100644
--- a/sphinx/conf.py
+++ b/sphinx/conf.py
@@ -50,7 +50,6 @@
     'cylc.sphinx_ext.cylc_lang',
     'cylc.sphinx_ext.diff_selection',
     'cylc.sphinx_ext.grid_table',
-    'cylc.sphinx_ext.hieroglyph_addons',
     'cylc.sphinx_ext.minicylc',
     'cylc.sphinx_ext.practical',
     'cylc.sphinx_ext.rtd_theme_addons',