diff --git a/doc/source/conf.py b/doc/source/conf.py
index 26492cc..131a315 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -34,7 +34,6 @@ extensions = [
     'sphinx.ext.intersphinx',
     'sphinx.ext.todo',
     'sphinx.ext.coverage',
-    'sphinx.ext.pngmath',
     'sphinx.ext.ifconfig',
     'sphinx.ext.viewcode',
 ]