diff --git a/gtsam/hybrid/HybridGaussianProductFactor.cpp b/gtsam/hybrid/HybridGaussianProductFactor.cpp
index fc61e1c54..30e1c57af 100644
--- a/gtsam/hybrid/HybridGaussianProductFactor.cpp
+++ b/gtsam/hybrid/HybridGaussianProductFactor.cpp
@@ -22,7 +22,7 @@
 #include <gtsam/hybrid/HybridGaussianProductFactor.h>
 #include <gtsam/linear/GaussianFactorGraph.h>
 
-#include <sstream>
+#include <string>
 
 namespace gtsam {