diff --git a/gtsam/hybrid/GaussianMixture.cpp b/gtsam/hybrid/GaussianMixture.cpp
index b5af0bf7f..e690af51c 100644
--- a/gtsam/hybrid/GaussianMixture.cpp
+++ b/gtsam/hybrid/GaussianMixture.cpp
@@ -35,7 +35,16 @@ GaussianMixture::GaussianMixture(
     : BaseFactor(CollectKeys(continuousFrontals, continuousParents),
                  discreteParents),
       BaseConditional(continuousFrontals.size()),
-      conditionals_(conditionals) {}
+      conditionals_(conditionals) {
+  // Calculate logConstant_ as the maximum of the log constants of the
+  // conditionals, by visiting the decision tree:
+  logConstant_ = -std::numeric_limits<double>::infinity();
+  conditionals_.visit(
+      [this](const GaussianConditional::shared_ptr &conditional) {
+        this->logConstant_ = std::max(this->logConstant_,
+                                      conditional->logNormalizationConstant());
+      });
+}
 
 /* *******************************************************************************/
 const GaussianMixture::Conditionals &GaussianMixture::conditionals() const {
@@ -203,8 +212,7 @@ boost::shared_ptr<GaussianMixtureFactor> GaussianMixture::likelihood(
   const KeyVector continuousParentKeys = continuousParents();
   const GaussianMixtureFactor::Factors likelihoods(
       conditionals_, [&](const GaussianConditional::shared_ptr &conditional) {
-        return GaussianMixtureFactor::sharedFactor{
-            conditional->likelihood(given)};
+        return conditional->likelihood(given);
       });
   return boost::make_shared<GaussianMixtureFactor>(
       continuousParentKeys, discreteParentKeys, likelihoods);
@@ -307,11 +315,23 @@ AlgebraicDecisionTree<Key> GaussianMixture::logProbability(
   return DecisionTree<Key, double>(conditionals_, errorFunc);
 }
 
+/* *******************************************************************************/
+AlgebraicDecisionTree<Key> GaussianMixture::error(
+    const VectorValues &continuousValues) const {
+  auto errorFunc = [&](const GaussianConditional::shared_ptr &conditional) {
+    return logConstant_ + conditional->error(continuousValues) -
+           conditional->logNormalizationConstant();
+  };
+  DecisionTree<Key, double> errorTree(conditionals_, errorFunc);
+  return errorTree;
+}
+
 /* *******************************************************************************/
 double GaussianMixture::error(const HybridValues &values) const {
   // Directly index to get the conditional, no need to build the whole tree.
   auto conditional = conditionals_(values.discrete());
-  return conditional->error(values.continuous()) - conditional->logNormalizationConstant();
+  return logConstant_ + conditional->error(values.continuous()) -
+         conditional->logNormalizationConstant();
 }
 
 /* *******************************************************************************/
diff --git a/gtsam/hybrid/GaussianMixture.h b/gtsam/hybrid/GaussianMixture.h
index 1b4e9126e..64eda218e 100644
--- a/gtsam/hybrid/GaussianMixture.h
+++ b/gtsam/hybrid/GaussianMixture.h
@@ -63,7 +63,8 @@ class GTSAM_EXPORT GaussianMixture
   using Conditionals = DecisionTree<Key, GaussianConditional::shared_ptr>;
 
  private:
-  Conditionals conditionals_;
+  Conditionals conditionals_;  ///< a decision tree of Gaussian conditionals.
+  double logConstant_;         ///< log of the normalization constant.
 
   /**
    * @brief Convert a DecisionTree of factors into a DT of Gaussian FGs.
@@ -155,6 +156,10 @@ class GTSAM_EXPORT GaussianMixture
   /// Returns the continuous keys among the parents.
   KeyVector continuousParents() const;
 
+  /// The log normalization constant is max of the the individual
+  /// log-normalization constants.
+  double logNormalizationConstant() const override { return logConstant_; }
+
   /// Return a discrete factor with possibly varying normalization constants.
   /// If there is no variation, return nullptr.
   boost::shared_ptr<DecisionTreeFactor> normalizationConstants() const;
@@ -192,18 +197,29 @@ class GTSAM_EXPORT GaussianMixture
    * in Conditional.h, should not depend on x, y, or m, only on the parameters
    * of the density. Hence, we delegate to the underlying Gaussian
    * conditionals, indexed by m, which do satisfy:
-   * 
+   *
    *    log(probability_m(x;y)) = K_m - error_m(x;y)
-   * 
-   * We resolve by having K == 0.0 and
-   * 
-   *    error(x;y,m) = error_m(x;y) - K_m
+   *
+   * We resolve by having K == max(K_m) and
+   *
+   *    error(x;y,m) = error_m(x;y) + K - K_m
+   *
+   * which also makes error(x;y,m) >= 0 for all x,y,m.
    *
    * @param values Continuous values and discrete assignment.
    * @return double
    */
   double error(const HybridValues &values) const override;
 
+  /**
+   * @brief Compute error of the GaussianMixture as a tree.
+   *
+   * @param continuousValues The continuous VectorValues.
+   * @return AlgebraicDecisionTree<Key> A decision tree on the discrete keys
+   * only, with the leaf values as the error for each assignment.
+   */
+  AlgebraicDecisionTree<Key> error(const VectorValues &continuousValues) const;
+
   /**
    * @brief Compute the logProbability of this Gaussian Mixture.
    *