diff --git a/tox.ini b/tox.ini
index 1ec0815b1..624f550e1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,7 +28,7 @@ deps =
     Pillow<2.0
     django==1.3.1
     cssselect==0.9.1
-    zope.interface=3.6.1
+    zope.interface==3.6.1
     -rtests/requirements.txt
 
 [testenv:trunk]