diff --git a/gtsam/hybrid/HybridFactor.cpp b/gtsam/hybrid/HybridFactor.cpp
index b25e97f05..89b1943cd 100644
--- a/gtsam/hybrid/HybridFactor.cpp
+++ b/gtsam/hybrid/HybridFactor.cpp
@@ -50,31 +50,37 @@ DiscreteKeys CollectDiscreteKeys(const DiscreteKeys &key1,
 
 /* ************************************************************************ */
 HybridFactor::HybridFactor(const KeyVector &keys)
-    : Base(keys), isContinuous_(true), continuousKeys_(keys) {}
+    : Base(keys),
+      category_(HybridCategory::Continuous),
+      continuousKeys_(keys) {}
 
 /* ************************************************************************ */
 HybridFactor::HybridFactor(const KeyVector &continuousKeys,
                            const DiscreteKeys &discreteKeys)
     : Base(CollectKeys(continuousKeys, discreteKeys)),
-      isDiscrete_((continuousKeys.size() == 0) && (discreteKeys.size() != 0)),
-      isContinuous_((continuousKeys.size() != 0) && (discreteKeys.size() == 0)),
-      isHybrid_((continuousKeys.size() != 0) && (discreteKeys.size() != 0)),
       discreteKeys_(discreteKeys),
-      continuousKeys_(continuousKeys) {}
+      continuousKeys_(continuousKeys) {
+  if ((continuousKeys.size() == 0) && (discreteKeys.size() != 0)) {
+    category_ = HybridCategory::Discrete;
+  } else if ((continuousKeys.size() != 0) && (discreteKeys.size() == 0)) {
+    category_ = HybridCategory::Continuous;
+  } else {
+    category_ = HybridCategory::Hybrid;
+  }
+}
 
 /* ************************************************************************ */
 HybridFactor::HybridFactor(const DiscreteKeys &discreteKeys)
     : Base(CollectKeys({}, discreteKeys)),
-      isDiscrete_(true),
+      category_(HybridCategory::Discrete),
       discreteKeys_(discreteKeys),
       continuousKeys_({}) {}
 
 /* ************************************************************************ */
 bool HybridFactor::equals(const HybridFactor &lf, double tol) const {
   const This *e = dynamic_cast<const This *>(&lf);
-  return e != nullptr && Base::equals(*e, tol) &&
-         isDiscrete_ == e->isDiscrete_ && isContinuous_ == e->isContinuous_ &&
-         isHybrid_ == e->isHybrid_ && continuousKeys_ == e->continuousKeys_ &&
+  return e != nullptr && Base::equals(*e, tol) && category_ == e->category_ &&
+         continuousKeys_ == e->continuousKeys_ &&
          discreteKeys_ == e->discreteKeys_;
 }
 
@@ -82,9 +88,18 @@ bool HybridFactor::equals(const HybridFactor &lf, double tol) const {
 void HybridFactor::print(const std::string &s,
                          const KeyFormatter &formatter) const {
   std::cout << (s.empty() ? "" : s + "\n");
-  if (isContinuous_) std::cout << "Continuous ";
-  if (isDiscrete_) std::cout << "Discrete ";
-  if (isHybrid_) std::cout << "Hybrid ";
+  switch (category_) {
+    case HybridCategory::Continuous:
+      std::cout << "Continuous ";
+      break;
+    case HybridCategory::Discrete:
+      std::cout << "Discrete ";
+      break;
+    case HybridCategory::Hybrid:
+      std::cout << "Hybrid ";
+      break;
+  }
+
   std::cout << "[";
   for (size_t c = 0; c < continuousKeys_.size(); c++) {
     std::cout << formatter(continuousKeys_.at(c));
diff --git a/gtsam/hybrid/HybridFactor.h b/gtsam/hybrid/HybridFactor.h
index c66116512..2cc7453f4 100644
--- a/gtsam/hybrid/HybridFactor.h
+++ b/gtsam/hybrid/HybridFactor.h
@@ -41,6 +41,9 @@ KeyVector CollectKeys(const KeyVector &keys1, const KeyVector &keys2);
 DiscreteKeys CollectDiscreteKeys(const DiscreteKeys &key1,
                                  const DiscreteKeys &key2);
 
+/// Enum to help with categorizing hybrid factors.
+enum class HybridCategory { Discrete, Continuous, Hybrid };
+
 /**
  * Base class for *truly* hybrid probabilistic factors
  *
@@ -53,9 +56,8 @@ DiscreteKeys CollectDiscreteKeys(const DiscreteKeys &key1,
  */
 class GTSAM_EXPORT HybridFactor : public Factor {
  private:
-  bool isDiscrete_ = false;
-  bool isContinuous_ = false;
-  bool isHybrid_ = false;
+  /// Record what category of HybridFactor this is.
+  HybridCategory category_;
 
  protected:
   // Set of DiscreteKeys for this factor.
@@ -116,13 +118,13 @@ class GTSAM_EXPORT HybridFactor : public Factor {
   /// @{
 
   /// True if this is a factor of discrete variables only.
-  bool isDiscrete() const { return isDiscrete_; }
+  bool isDiscrete() const { return category_ == HybridCategory::Discrete; }
 
   /// True if this is a factor of continuous variables only.
-  bool isContinuous() const { return isContinuous_; }
+  bool isContinuous() const { return category_ == HybridCategory::Continuous; }
 
   /// True is this is a Discrete-Continuous factor.
-  bool isHybrid() const { return isHybrid_; }
+  bool isHybrid() const { return category_ == HybridCategory::Hybrid; }
 
   /// Return the number of continuous variables in this factor.
   size_t nrContinuous() const { return continuousKeys_.size(); }
@@ -142,9 +144,7 @@ class GTSAM_EXPORT HybridFactor : public Factor {
   template <class ARCHIVE>
   void serialize(ARCHIVE &ar, const unsigned int /*version*/) {
     ar &BOOST_SERIALIZATION_BASE_OBJECT_NVP(Base);
-    ar &BOOST_SERIALIZATION_NVP(isDiscrete_);
-    ar &BOOST_SERIALIZATION_NVP(isContinuous_);
-    ar &BOOST_SERIALIZATION_NVP(isHybrid_);
+    ar &BOOST_SERIALIZATION_NVP(category_);
     ar &BOOST_SERIALIZATION_NVP(discreteKeys_);
     ar &BOOST_SERIALIZATION_NVP(continuousKeys_);
   }
diff --git a/gtsam/hybrid/tests/testHybridBayesNet.cpp b/gtsam/hybrid/tests/testHybridBayesNet.cpp
index cf4231dba..99487a84a 100644
--- a/gtsam/hybrid/tests/testHybridBayesNet.cpp
+++ b/gtsam/hybrid/tests/testHybridBayesNet.cpp
@@ -387,11 +387,13 @@ TEST(HybridBayesNet, Sampling) {
       std::make_shared<BetweenFactor<double>>(X(0), X(1), 0, noise_model);
   auto one_motion =
       std::make_shared<BetweenFactor<double>>(X(0), X(1), 1, noise_model);
-  std::vector<NonlinearFactorValuePair> factors = {{zero_motion, 0.0},
-                                                   {one_motion, 0.0}};
+
+  DiscreteKeys discreteKeys{DiscreteKey(M(0), 2)};
+  HybridNonlinearFactor::Factors factors(
+      discreteKeys, {{zero_motion, 0.0}, {one_motion, 0.0}});
   nfg.emplace_shared<PriorFactor<double>>(X(0), 0.0, noise_model);
-  nfg.emplace_shared<HybridNonlinearFactor>(
-      KeyVector{X(0), X(1)}, DiscreteKeys{DiscreteKey(M(0), 2)}, factors);
+  nfg.emplace_shared<HybridNonlinearFactor>(KeyVector{X(0), X(1)}, discreteKeys,
+                                            factors);
 
   DiscreteKey mode(M(0), 2);
   nfg.emplace_shared<DiscreteDistribution>(mode, "1/1");