diff --git a/gtsam/hybrid/HybridFactorGraph.cpp b/gtsam/hybrid/HybridFactorGraph.cpp
index 4238925d6..098942f10 100644
--- a/gtsam/hybrid/HybridFactorGraph.cpp
+++ b/gtsam/hybrid/HybridFactorGraph.cpp
@@ -25,17 +25,17 @@
 namespace gtsam {
 
 /* ************************************************************************* */
-DiscreteKeys HybridFactorGraph::discreteKeys() const {
-  DiscreteKeys keys;
+std::set<DiscreteKey> HybridFactorGraph::discreteKeys() const {
+  std::set<DiscreteKey> keys;
   for (auto& factor : factors_) {
     if (auto p = boost::dynamic_pointer_cast<DecisionTreeFactor>(factor)) {
       for (const DiscreteKey& key : p->discreteKeys()) {
-        keys.push_back(key);
+        keys.insert(key);
       }
     }
     if (auto p = boost::dynamic_pointer_cast<HybridFactor>(factor)) {
       for (const DiscreteKey& key : p->discreteKeys()) {
-        keys.push_back(key);
+        keys.insert(key);
       }
     }
   }
diff --git a/gtsam/hybrid/HybridFactorGraph.h b/gtsam/hybrid/HybridFactorGraph.h
index 7d30663a3..a02d4a212 100644
--- a/gtsam/hybrid/HybridFactorGraph.h
+++ b/gtsam/hybrid/HybridFactorGraph.h
@@ -65,7 +65,7 @@ class HybridFactorGraph : public FactorGraph<Factor> {
   /// @{
 
   /// Get all the discrete keys in the factor graph.
-  DiscreteKeys discreteKeys() const;
+  std::set<DiscreteKey> discreteKeys() const;
 
   /// Get all the discrete keys in the factor graph, as a set.
   KeySet discreteKeySet() const;