diff --git a/.readthedocs.yml b/.readthedocs.yml
index ccc19dd..4947f2f 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -1,32 +1,32 @@
 # .readthedocs.yml
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 
 # Required
 version: 2
 
 build:
   os: ubuntu-20.04
   tools:
-    python: "3"
+    python: "3.9"
   apt_packages:
     - inkscape
     - imagemagick
     - graphviz
 
 
 # Build documentation in the doc/ directory with Sphinx
 sphinx:
   configuration: doc/sphinx/source/conf.py
 
 # Build documentation with MkDocs
 #mkdocs:
 #  configuration: mkdocs.yml
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
   install:
     - requirements: doc/requirements.txt