diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 28ebf6745..f3ec2a34e 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -890,16 +890,18 @@ class FederationSenderHandler(object):
             # we're not being re-entered?
 
             with (await self._fed_position_linearizer.queue(None)):
+                # We persist and ack the same position, so we take a copy of it
+                # here as otherwise it can get modified from underneath us.
+                current_position = self.federation_position
+
                 await self.store.update_federation_out_pos(
-                    "federation", self.federation_position
+                    "federation", current_position
                 )
 
                 # We ACK this token over replication so that the master can drop
                 # its in memory queues
-                self._hs.get_tcp_replication().send_federation_ack(
-                    self.federation_position
-                )
-                self._last_ack = self.federation_position
+                self._hs.get_tcp_replication().send_federation_ack(current_position)
+                self._last_ack = current_position
         except Exception:
             logger.exception("Error updating federation stream position")