diff --git a/gtsam/hybrid/HybridBayesTree.cpp b/gtsam/hybrid/HybridBayesTree.cpp
index 0766f452b..df4a49f60 100644
--- a/gtsam/hybrid/HybridBayesTree.cpp
+++ b/gtsam/hybrid/HybridBayesTree.cpp
@@ -27,6 +27,7 @@
 #include <gtsam/linear/GaussianJunctionTree.h>
 
 #include <memory>
+#include <string>
 
 #include "gtsam/hybrid/HybridConditional.h"