diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 454c3d4ab..b5e201b5f 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -148,7 +148,7 @@ class MatrixFederationHttpClient(object):
 
                 if hasattr(e, "reasons"):
                     reasons = ", ".join(
-                        f.value.message
+                        str(f.value.message)
                         for f in e.reasons
                     )
                 else: