diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 1c7aeea..33c2a61 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -9,6 +9,7 @@
 version: 2
 
 build:
+  os: ubuntu-20.04
   tools:
     python: "3"