diff --git a/gtsam/nonlinear/FixedLagSmoother.h b/gtsam/nonlinear/FixedLagSmoother.h
index ba17b474b..f628815a4 100644
--- a/gtsam/nonlinear/FixedLagSmoother.h
+++ b/gtsam/nonlinear/FixedLagSmoother.h
@@ -20,6 +20,10 @@
 // \callgraph
 #pragma once
 
+#ifdef GTSAN_BUILD_UNSTABLE
+#include <gtsam_unstable/dllexport.h>
+#endif
+
 #include <gtsam/dllexport.h>
 #include <gtsam/inference/Key.h>
 #include <gtsam/nonlinear/NonlinearFactorGraph.h>