diff --git a/doc/conf.py b/doc/conf.py
index 96425f5ca36ff47d647ad0d33baf6e14b5cb32a6..90254c22f1cee57d5fc65a2cdfe395e0a27f3a93 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -33,6 +33,7 @@ date = datetime.date.today()
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
+    'sphinx.ext.viewcode',
     'sphinx.ext.autosummary',
     'sphinx.ext.mathjax',
     'sphinx.ext.graphviz',