diff --git a/datasette/publish/heroku.py b/datasette/publish/heroku.py
index 0f8564c3..0aa29feb 100644
--- a/datasette/publish/heroku.py
+++ b/datasette/publish/heroku.py
@@ -167,7 +167,7 @@ def temporary_heroku_directory(
         if metadata_content:
             open("metadata.json", "w").write(json.dumps(metadata_content, indent=2))
 
-        open("runtime.txt", "w").write("python-3.8.5")
+        open("runtime.txt", "w").write("python-3.8.6")
 
         if branch:
             install = [