diff --git a/tox.ini b/tox.ini
index 9db1f95..c29d216 100644
--- a/tox.ini
+++ b/tox.ini
@@ -49,7 +49,7 @@ commands =
     rm -rf api-ref/build api-ref/html
     rm -rf doc/source/apidoc doc/source/api
     python setup.py build_sphinx
-    sphinx-build -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
+    sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 whitelist_externals =
     rm
 
@@ -57,7 +57,7 @@ whitelist_externals =
 install_command = pip install -U --force-reinstall {opts} {packages}
 commands =
   rm -rf api-ref/build api-ref/html
-  sphinx-build -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
+  sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 whitelist_externals =
     rm