diff --git a/docs/requirements.txt b/docs/requirements.txt
index 3b229ab1..a5274531 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,4 +1,6 @@
 Pygments >= 2.18.0
 Sphinx == 5.0.2
+standard-imghdr
+  # This is needed for old Sphinges on new Pythons.
 sphinx_rtd_theme == 1.2.2
 git+https://github.com/hylang/sphinxcontrib-hydomain.git