fix_interop_server_arg
diff --git a/tools/run_tests/run_interop_tests.py b/tools/run_tests/run_interop_tests.py
index 78096b2..0d402d6 100755
--- a/tools/run_tests/run_interop_tests.py
+++ b/tools/run_tests/run_interop_tests.py
@@ -662,7 +662,7 @@
'cloud_to_prod_auth tests against.'))
argp.add_argument('-s', '--server',
choices=['all'] + sorted(_SERVERS),
- action='append',
+ nargs='+',
help='Run cloud_to_cloud servers in a separate docker ' +
'image. Servers can only be started automatically if ' +
'--use_docker option is enabled.',