diff --git a/gtsam/discrete/DiscreteKey.cpp b/gtsam/discrete/DiscreteKey.cpp
index 79d11d8a7..17f233695 100644
--- a/gtsam/discrete/DiscreteKey.cpp
+++ b/gtsam/discrete/DiscreteKey.cpp
@@ -49,6 +49,7 @@ namespace gtsam {
 
   void DiscreteKeys::print(const std::string& s,
                            const KeyFormatter& keyFormatter) const {
+    std::cout << (s.empty() ? "" : s + " ") << std::endl;
     for (auto&& dkey : *this) {
       std::cout << DefaultKeyFormatter(dkey.first) << " " << dkey.second
                 << std::endl;