diff --git a/libmproxy/cmdline.py b/libmproxy/cmdline.py
index 9f8142a0f..57fce2c53 100644
--- a/libmproxy/cmdline.py
+++ b/libmproxy/cmdline.py
@@ -275,9 +275,9 @@ def common_options(parser):
         help="Replacement pattern, where the replacement clause is a path to a file."
     )
 
-    group.add_option(
+    group.add_argument(
         "--dummy-certs", action="store",
-        type = "str", dest = "certdir", default=None,
+        type = str, dest = "certdir", default=None,
         help = "Generated dummy certs directory."
     )