diff --git a/setup.py b/setup.py
index 4a7b3c99aa..0e3f2f18d5 100755
--- a/setup.py
+++ b/setup.py
@@ -65,6 +65,8 @@ class Tox(Command):
         args = self.tox_args
         if args:
             args = shlex.split(self.tox_args)
+        else:
+            args = []
         errno = tox.cmdline(args=args)
         sys.exit(errno)