diff --git a/sslyze/connection_helpers/tls_connection.py b/sslyze/connection_helpers/tls_connection.py
index e86ea211..3b7a5475 100644
--- a/sslyze/connection_helpers/tls_connection.py
+++ b/sslyze/connection_helpers/tls_connection.py
@@ -127,6 +127,7 @@ def _open_socket(server_location: ServerNetworkLocation, network_timeout: int) -
     # enabled in the client; for example client only supports EC cipher suites but server returned an RSA certificate
     "wrong certificate type": "Server returned wrong certificate type",
     "invalid encoding": "TLS error: Invalid encoding",
+    "certificate unknown": "TLS alert: certificate unknown"
 }