diff --git a/tox.ini b/tox.ini
index 9d9f0a50..2ec91094 100644
--- a/tox.ini
+++ b/tox.ini
@@ -31,7 +31,7 @@ deps =
 commands =
     bash -c "rm -rf doc/build"
     doc8 doc
-    sphinx-build --keep-going -b html doc/source doc/build/html
+    sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}