diff --git a/doc/source/api/README b/doc/source/api/README
index 3bb628f5..643cd1e0 100644
--- a/doc/source/api/README
+++ b/doc/source/api/README
@@ -1 +1,4 @@
 Don't put files in here, it's intended only for auto-generated output!
+
+Specifically, files which aren't known to git will be cleaned out of
+this directory before the docs are generated by tox.
diff --git a/tox.ini b/tox.ini
index 9621c452..7cddb181 100644
--- a/tox.ini
+++ b/tox.ini
@@ -25,7 +25,9 @@ commands = {envpython} {toxinidir}/tools/noproxy nosetests --with-coverage --cov
 commands = flake8 {posargs}
 
 [testenv:docs]
+whitelist_externals = git
 commands =
+    git clean -fx doc/source/api
     python setup.py build_sphinx
     doc8 doc/source