diff --git a/tox.ini b/tox.ini
index d1f17e5..7d538a7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,7 @@ deps =
     wheel
     pytest==3.2.5
     coverage
-    pytest-cov
+    pytest-cov==2.5.1
     stdeb
 commands =
     py.test --cov=dpkt dpkt