diff --git a/tests/utils.py b/tests/utils.py
index 4dbe201..965338a 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -45,7 +45,7 @@ async def set_up_nodes_by_transport_and_disc_opt(transport_disc_opt_list):
     return tuple(nodes_list)
 
 
-async def set_up_routers(router_confs=[0, 0]):
+async def set_up_routers(router_confs=(0, 0)):
     """The default ``router_confs`` selects two free ports local to this
     machine."""
     bootstrap_node = KademliaServer()