diff --git a/timely/src/progress/subgraph.rs b/timely/src/progress/subgraph.rs
index 2babf5d27..1b7bdb0d8 100644
--- a/timely/src/progress/subgraph.rs
+++ b/timely/src/progress/subgraph.rs
@@ -547,6 +547,7 @@ where
     }
 
     fn set_external_summary(&mut self) {
+        self.accept_frontier();
         self.propagate_pointstamps();  // ensure propagation of input frontiers.
         self.children
             .iter_mut()