diff --git a/docs/rtfd-requirements.txt b/docs/rtfd-requirements.txt
index d02c7aa..8448ce7 100644
--- a/docs/rtfd-requirements.txt
+++ b/docs/rtfd-requirements.txt
@@ -1,6 +1,6 @@
 attrs==19.1.0
 cffi==1.12.2
-Sphinx==1.8.4
+Sphinx==4.1.2
 sphinx-rtd-theme==0.4.3
 sphinxcontrib-websupport==1.1.0
 trio==0.11.0