diff --git a/docs/subtract_three.rst b/docs/subtract_three.rst
index b13694c..d8328fc 100644
--- a/docs/subtract_three.rst
+++ b/docs/subtract_three.rst
@@ -1,5 +1,5 @@
 Subtract three
-========
+==============
 
 Example call
 ------------
@@ -14,6 +14,6 @@ To decrement the value of a number by three, use the `subtract_three` function.
 
 
 Description of the `subtract_three` function:
----------------------------------------
+---------------------------------------------
 
 .. autofunction:: python_package_template.subtract_three
diff --git a/tox.ini b/tox.ini
index d8d9209..96274f5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -35,4 +35,5 @@ commands =
 extras =
     docs
 commands =
-    sphinx-build -b html -d "{envtmpdir}{/}doctree" docs "{toxworkdir}{/}docs_out" --color
+    sphinx-build --builder html --doctree-dir "{envtmpdir}{/}doctree" \
+    docs "{toxworkdir}{/}docs_out" --color --fail-on-warning