diff --git a/gtsam/inference/FactorGraph-inl.h b/gtsam/inference/FactorGraph-inl.h
index f757f5e74..60fc2e767 100644
--- a/gtsam/inference/FactorGraph-inl.h
+++ b/gtsam/inference/FactorGraph-inl.h
@@ -22,6 +22,7 @@
 
 #pragma once
 
+#include <gtsam/base/FastSet.h>
 #include <gtsam/inference/BayesTree.h>
 #include <gtsam/inference/VariableIndex.h>