diff --git a/tox.ini b/tox.ini
index 95fee06..fcfc628 100644
--- a/tox.ini
+++ b/tox.ini
@@ -18,7 +18,9 @@ passenv = WITH_GCOV
 commands = {envpython} -bb -Werror \
     "-Wignore:the imp module is deprecated:DeprecationWarning" \
     "-Wignore:the imp module is deprecated:PendingDeprecationWarning" \
-    -m coverage run --parallel setup.py test
+    -m coverage run --parallel setup.py \
+        clean --all \
+        test
 
 [testenv:py27]
 # No warnings with Python 2.7