diff --git a/book/_toc.yml b/book/_toc.yml
index 92aa83d..9c9bb20 100644
--- a/book/_toc.yml
+++ b/book/_toc.yml
@@ -60,7 +60,7 @@ parts:
         sections:
         - file: features/widgets.ipynb
         - file: features/exercise_checking.ipynb
-      -file: external/Sphinx-Image-Inverter/README.md
+      - file: external/Sphinx-Image-Inverter/README.md
 
   - caption: Examples
     chapters: