diff --git a/book/_config.yml b/book/_config.yml
index 21a4bbc..c8d247f 100644
--- a/book/_config.yml
+++ b/book/_config.yml
@@ -21,7 +21,7 @@ sphinx:
   - sphinx_exercise
   - sphinx_proof
   - sphinx_named_colors
-  - sphinx-iframes
+  - sphinx_iframes
   config:
     html_js_files:
     - https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.4/require.min.js