diff --git a/gtsam/hybrid/HybridGaussianProductFactor.cpp b/gtsam/hybrid/HybridGaussianProductFactor.cpp
index e71201ceb..fc61e1c54 100644
--- a/gtsam/hybrid/HybridGaussianProductFactor.cpp
+++ b/gtsam/hybrid/HybridGaussianProductFactor.cpp
@@ -22,6 +22,8 @@
 #include <gtsam/hybrid/HybridGaussianProductFactor.h>
 #include <gtsam/linear/GaussianFactorGraph.h>
 
+#include <sstream>
+
 namespace gtsam {
 
 using Y = GaussianFactorGraphValuePair;