diff --git a/libmproxy/utils.py b/libmproxy/utils.py
index f4df813f9..34c49e14c 100644
--- a/libmproxy/utils.py
+++ b/libmproxy/utils.py
@@ -411,6 +411,7 @@ def dummy_cert(certdir, ca, commonname):
             "-days", "9999",
             "-out", certpath,
             "-CA", ca,
+            "-CAcreateserial",
             "-extfile", confpath,
             "-extensions", "v3_cert",
         ]