diff --git a/tox.ini b/tox.ini
index 9cf8bf7c..f95c0c71 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,7 +4,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-commands = py.test {posargs:tests/}
+commands = py.test {posargs:tests/} --assert=plain
 deps =
   -rrequirements_test.txt
   py26: unittest2