diff --git a/synapse/crypto/context_factory.py b/synapse/crypto/context_factory.py
index 9667b0c806..fbe2bd454d 100644
--- a/synapse/crypto/context_factory.py
+++ b/synapse/crypto/context_factory.py
@@ -130,7 +130,7 @@ class ClientTLSOptionsFactory(object):
         self._config = config
         self._options_validate = CertificateOptions(
             # This option implies verify=True
-            trustRoot=self._config.federation_custom_ca_list,
+            trustRoot=config.federation_custom_ca_list,
         )
         self._options_novalidate = CertificateOptions(verify=False)