diff --git a/changes.d/6397.fix.md b/changes.d/6397.fix.md
new file mode 100644
index 0000000000..589c29dbb1
--- /dev/null
+++ b/changes.d/6397.fix.md
@@ -0,0 +1 @@
+Fix "dictionary changed size during iteration error" which could occur with broadcasts.
diff --git a/cylc/flow/data_store_mgr.py b/cylc/flow/data_store_mgr.py
index bfadadfb55..3b49050f0f 100644
--- a/cylc/flow/data_store_mgr.py
+++ b/cylc/flow/data_store_mgr.py
@@ -2251,7 +2251,9 @@ def delta_broadcast(self):
 
     def _generate_broadcast_node_deltas(self, node_data, node_type):
         cfg = self.schd.config.cfg
-        for node_id, node in node_data.items():
+        # NOTE: node_data may change during operation so make a copy
+        # see https://github.com/cylc/cylc-flow/pull/6397
+        for node_id, node in list(node_data.items()):
             tokens = Tokens(node_id)
             new_runtime = runtime_from_config(
                 self._apply_broadcasts_to_runtime(