diff --git a/gtsam/inference/ClusterTree.h b/gtsam/inference/ClusterTree.h
index 7544587c1..c5301f030 100644
--- a/gtsam/inference/ClusterTree.h
+++ b/gtsam/inference/ClusterTree.h
@@ -21,6 +21,7 @@
 
 #include <list>
 #include <vector>
+#include <string>
 
 #include <boost/shared_ptr.hpp>
 #include <boost/weak_ptr.hpp>