diff --git a/gtsam/hybrid/HybridConditional.h b/gtsam/hybrid/HybridConditional.h
index 588c44e0b..3921df170 100644
--- a/gtsam/hybrid/HybridConditional.h
+++ b/gtsam/hybrid/HybridConditional.h
@@ -25,6 +25,7 @@
 #include <gtsam/inference/Key.h>
 #include <gtsam/linear/GaussianConditional.h>
 
+#include <iostream>
 #include <memory>
 #include <stdexcept>
 #include <string>