diff --git a/synapse/federation/transaction_queue.py b/synapse/federation/transaction_queue.py
index 741a4e7a1a..e6eb85501a 100644
--- a/synapse/federation/transaction_queue.py
+++ b/synapse/federation/transaction_queue.py
@@ -144,7 +144,7 @@ class TransactionQueue(object):
                 deferred.errback(failure)
 
         def log_failure(failure):
-            logger.warn("Failed to send pdu", failure.value)
+            logger.warn("Failed to send pdu: %s", failure.value)
 
         deferred.addErrback(log_failure)