diff --git a/gtsam/hybrid/HybridBayesNet.h b/gtsam/hybrid/HybridBayesNet.h
index bba301be2..f46ce5249 100644
--- a/gtsam/hybrid/HybridBayesNet.h
+++ b/gtsam/hybrid/HybridBayesNet.h
@@ -25,6 +25,8 @@
 #include <gtsam/inference/BayesNet.h>
 #include <gtsam/linear/GaussianBayesNet.h>
 
+#include <string>
+
 namespace gtsam {
 
 /**