diff --git a/tox.ini b/tox.ini
index cd7a757..db53159 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,12 +2,13 @@
 skipsdist=true
 
 [testenv]
+whitelist_externals = bash
 commands =
     {envpython} -V
     {envpython} -m compileall speedtest.py
     {envpython} speedtest.py
     {envpython} speedtest.py --source 172.17.0.1
-    bash -c \'{envpython} speedtest.py --source 127.0.0.1 || true\'
+    bash -c \'{envpython} speedtest.py --source 127.0.0.1 | grep "Invalid argument"\'
 
 [testenv:flake8]
 basepython=python
@@ -17,9 +18,10 @@ commands =
     flake8 speedtest.py
 
 [testenv:pypy]
+whitelist_externals = bash
 commands =
     pypy -V
     pypy -m compileall speedtest.py
     pypy speedtest.py
     pypy speedtest.py --source 172.17.0.1
-    bash -c \'pypy speedtest.py --source 127.0.0.1 || true\'
+    bash -c \'pypy speedtest.py --source 127.0.0.1 | grep "Invalid argument"\'