diff --git a/.travis.yml b/.travis.yml
index fc5518a1f8..a33d822147 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -78,9 +78,9 @@ matrix:
             firefox: 57.0
         - python: 3.5
           env: GROUP=python
-        - python: 3.7
-          dist: xenial
+        - python: "3.7"
           env: GROUP=python
+          dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
         - python: 3.6
           env: GROUP=docs