diff --git a/readthedocs.yml b/readthedocs.yml
index 313b6e0552..e7467d2df7 100644
--- a/readthedocs.yml
+++ b/readthedocs.yml
@@ -3,7 +3,7 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: 3.9
+    python: "3.9"
 
 python:
   install: