diff --git a/.travis.yml b/.travis.yml
index 16fd025..0bc552a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,7 +28,9 @@
         - TESTING=docs
       before_script:
         - cd Doc
-        - pip install -U Sphinx
+        # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
+        # (Updating the version is fine as long as no warnings are raised by doing so.)
+        - python -m pip install sphinx~=1.5.6
       script:
         - make check suspicious html SPHINXOPTS="-q -W"