diff --git a/tox.ini b/tox.ini
index d7eb9144..758aa761 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,6 +8,8 @@ deps =
     py34: typing
 # numpydoc for typing scipy stack
     numpydoc
+# sphinx, a dependency of numpydoc, dropped Python 2 support in version 2.0
+    sphinx < 2.0
     cov: coverage
 # Overwrite the parso version (only used sometimes).
 #     git+https://github.com/davidhalter/parso.git