diff --git a/tox.ini b/tox.ini
index f6ace8dd5..9dcbbf76c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -23,6 +23,8 @@ deps =
 
 [testenv:docs]
 changedir = docs
+deps =
+    sphinx
 commands =
     sphinx-build -W -b html . {envtmpdir}/html