diff --git a/requirements.txt b/requirements.txt
index 92658c4..8ac5bfb 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,6 +9,7 @@ sphinx-proof == 0.2.1
 sphinx-named-colors == 1.0.1
 sphinx-iframes == 1.0.5
 sphinx-prime-applets == 1.0.2
+sphinx-dropdown-toggle == 0.0.2
 
 git+https://github.com/TeachBooks/Sphinx-Thebe