diff --git a/changelog.d/14280.bugfix b/changelog.d/14280.bugfix
new file mode 100644
index 0000000000..c546d2be48
--- /dev/null
+++ b/changelog.d/14280.bugfix
@@ -0,0 +1 @@
+Fix broken outbound federation when using Python 3.7. Broke in v1.70.0rc1.
diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index 774ecd81b6..3ad483efe0 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -536,8 +536,7 @@ class FederationSender(AbstractFederationSender):
 
                 if event_entries:
                     now = self.clock.time_msec()
-                    last_id = next(reversed(event_ids))
-                    ts = event_to_received_ts[last_id]
+                    ts = max(t for t in event_to_received_ts.values() if t)
                     assert ts is not None
 
                     synapse.metrics.event_processing_lag.labels(