diff --git a/gtsam_unstable/nonlinear/IncrementalFixedLagSmoother.cpp b/gtsam_unstable/nonlinear/IncrementalFixedLagSmoother.cpp
index 52e56260d6..ec543e45fc 100644
--- a/gtsam_unstable/nonlinear/IncrementalFixedLagSmoother.cpp
+++ b/gtsam_unstable/nonlinear/IncrementalFixedLagSmoother.cpp
@@ -118,6 +118,10 @@ FixedLagSmoother::Result IncrementalFixedLagSmoother::update(
   std::set<Key> additionalKeys;
   for(Key key: marginalizableKeys) {
     ISAM2Clique::shared_ptr clique = isam_[key];
+    // Mark all frontal keys of the current clique.
+    additionalKeys.insert(clique->conditional()->frontals().begin(),
+                          clique->conditional()->frontals().end());
+    // Recursively mark all of the children key that contain the marginal key.
     for(const ISAM2Clique::shared_ptr& child: clique->children) {
       recursiveMarkAffectedKeys(key, child, additionalKeys);
     }