diff --git a/gtsam/inference/VariableIndexUnordered.h b/gtsam/inference/VariableIndexUnordered.h
index ada3a206e..140f51d40 100644
--- a/gtsam/inference/VariableIndexUnordered.h
+++ b/gtsam/inference/VariableIndexUnordered.h
@@ -56,6 +56,7 @@ protected:
 public:
   typedef KeyMap::const_iterator const_iterator;
   typedef KeyMap::const_iterator iterator;
+  typedef KeyMap::value_type value_type;
 
 public: