diff --git a/libmproxy/proxy/config.py b/libmproxy/proxy/config.py
index 4ae93368a..0a1345fa0 100644
--- a/libmproxy/proxy/config.py
+++ b/libmproxy/proxy/config.py
@@ -13,7 +13,7 @@ class ProxyConfig:
     def __init__(self, confdir=CONF_DIR, clientcerts=None,
                        no_upstream_cert=False, body_size_limit=None, get_upstream_server=None,
                        http_form_in="absolute", http_form_out="relative", authenticator=None,
-                       ciphers=None, certs=None, certforward = False
+                       ciphers=None, certs=[], certforward = False
                 ):
         self.ciphers = ciphers
         self.clientcerts = clientcerts