diff --git a/tox.ini b/tox.ini
index 60517e2c..f4e4e4c8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -48,6 +48,7 @@ deps =
     coverage
     coveralls==1.2.0
     pycparser==2.18
+    idna==2.7
 commands = {[coverage]commands}
 
 [testenv:pypy]