diff --git a/tox.ini b/tox.ini
index 4d44c48eaa3..5216e936e52 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,7 +17,7 @@ commands =
 
 setenv =
     debug: PYTHONASYNCIODEBUG = 1
-    pure: AIOHTTP+NO_EXTENSIONS = 1
+    pure: AIOHTTP_NO_EXTENSIONS = 1
 
 basepython:
     py33: python3.3