diff --git a/requirements.txt b/requirements.txt
index 071e3d6..f259806 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ Flask==1.1.2
 numpy
 google-cloud-firestore
 scikit-surgerycore>0.6.5
+sphinx<3.4.1