diff --git a/distributed/worker.py b/distributed/worker.py
index 821b3084a86..34323cb4584 100644
--- a/distributed/worker.py
+++ b/distributed/worker.py
@@ -945,6 +945,8 @@ async def heartbeat(self):
         logger.debug("Heartbeat: %s", self.address)
         try:
             start = time()
+            with self.active_threads_lock:
+                active_keys = list(self.active_threads.values())
             response = await retry_operation(
                 self.scheduler.heartbeat_worker,
                 address=self.contact_address,
@@ -952,7 +954,7 @@ async def heartbeat(self):
                 metrics=await self.get_metrics(),
                 executing={
                     key: start - self.tasks[key].start_time
-                    for key in self.active_threads.values()
+                    for key in active_keys
                     if key in self.tasks
                 },
             )