diff --git a/requirements.txt b/requirements.txt
index aeff4075..0d4e48a1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -12,6 +12,7 @@ attrs
 # ReadTheDocs indirectly depends on some too new extension versions.
 sphinxcontrib-applehelp<=1.0.4
 sphinxcontrib-devhelp<=1.0.5
+sphinxcontrib-htmlhelp<=2.0.4
 
 # Utility for easily running builds in Python virtual environments.
 venv-run