diff --git a/gtsam/hybrid/GaussianMixture.h b/gtsam/hybrid/GaussianMixture.h
index 4df1bd90c..a9b05f250 100644
--- a/gtsam/hybrid/GaussianMixture.h
+++ b/gtsam/hybrid/GaussianMixture.h
@@ -170,7 +170,7 @@ class GTSAM_EXPORT GaussianMixture
    * @param values Continuous values and discrete assignment.
    * @return double
    */
-  double error(const HybridValues &values) const;
+  double error(const HybridValues &values) const override;
 
   /**
    * @brief Prune the decision tree of Gaussian factors as per the discrete
diff --git a/gtsam/hybrid/GaussianMixtureFactor.h b/gtsam/hybrid/GaussianMixtureFactor.h
index a96f253ce..ce011fecc 100644
--- a/gtsam/hybrid/GaussianMixtureFactor.h
+++ b/gtsam/hybrid/GaussianMixtureFactor.h
@@ -165,7 +165,7 @@ class GTSAM_EXPORT GaussianMixtureFactor : public HybridFactor {
    * @brief Compute the log-likelihood, including the log-normalizing constant.
    * @return double
    */
-  double error(const HybridValues &values) const;
+  double error(const HybridValues &values) const override;
 
   /// Add MixtureFactor to a Sum, syntactic sugar.
   friend Sum &operator+=(Sum &sum, const GaussianMixtureFactor &factor) {
diff --git a/gtsam/hybrid/HybridConditional.h b/gtsam/hybrid/HybridConditional.h
index db03ba59c..be671d55f 100644
--- a/gtsam/hybrid/HybridConditional.h
+++ b/gtsam/hybrid/HybridConditional.h
@@ -52,7 +52,7 @@ namespace gtsam {
  * having diamond inheritances, and neutralized the need to change other
  * components of GTSAM to make hybrid elimination work.
  *
- * A great reference to the type-erasure pattern is Eduaado Madrid's CppCon
+ * A great reference to the type-erasure pattern is Eduardo Madrid's CppCon
  * talk (https://www.youtube.com/watch?v=s082Qmd_nHs).
  *
  * @ingroup hybrid
@@ -129,33 +129,6 @@ class GTSAM_EXPORT HybridConditional
    */
   HybridConditional(boost::shared_ptr<GaussianMixture> gaussianMixture);
 
-  /**
-   * @brief Return HybridConditional as a GaussianMixture
-   * @return nullptr if not a mixture
-   * @return GaussianMixture::shared_ptr otherwise
-   */
-  GaussianMixture::shared_ptr asMixture() {
-    return boost::dynamic_pointer_cast<GaussianMixture>(inner_);
-  }
-
-  /**
-   * @brief Return HybridConditional as a GaussianConditional
-   * @return nullptr if not a GaussianConditional
-   * @return GaussianConditional::shared_ptr otherwise
-   */
-  GaussianConditional::shared_ptr asGaussian() {
-    return boost::dynamic_pointer_cast<GaussianConditional>(inner_);
-  }
-
-  /**
-   * @brief Return conditional as a DiscreteConditional
-   * @return nullptr if not a DiscreteConditional
-   * @return DiscreteConditional::shared_ptr
-   */
-  DiscreteConditional::shared_ptr asDiscrete() {
-    return boost::dynamic_pointer_cast<DiscreteConditional>(inner_);
-  }
-
   /// @}
   /// @name Testable
   /// @{
@@ -169,10 +142,52 @@ class GTSAM_EXPORT HybridConditional
   bool equals(const HybridFactor& other, double tol = 1e-9) const override;
 
   /// @}
+  /// @name Standard Interface
+  /// @{
+
+  /**
+   * @brief Return HybridConditional as a GaussianMixture
+   * @return nullptr if not a mixture
+   * @return GaussianMixture::shared_ptr otherwise
+   */
+  GaussianMixture::shared_ptr asMixture() const {
+    return boost::dynamic_pointer_cast<GaussianMixture>(inner_);
+  }
+
+  /**
+   * @brief Return HybridConditional as a GaussianConditional
+   * @return nullptr if not a GaussianConditional
+   * @return GaussianConditional::shared_ptr otherwise
+   */
+  GaussianConditional::shared_ptr asGaussian() const {
+    return boost::dynamic_pointer_cast<GaussianConditional>(inner_);
+  }
+
+  /**
+   * @brief Return conditional as a DiscreteConditional
+   * @return nullptr if not a DiscreteConditional
+   * @return DiscreteConditional::shared_ptr
+   */
+  DiscreteConditional::shared_ptr asDiscrete() const {
+    return boost::dynamic_pointer_cast<DiscreteConditional>(inner_);
+  }
 
   /// Get the type-erased pointer to the inner type
   boost::shared_ptr<Factor> inner() { return inner_; }
 
+  /// Return the error of the underlying conditional.
+  /// Currently only implemented for Gaussian mixture.
+  double error(const HybridValues& values) const override {
+    if (auto gm = asMixture()) {
+      return gm->error(values);
+    } else {
+      throw std::runtime_error(
+          "HybridConditional::error: only implemented for Gaussian mixture");
+    }
+  }
+
+  /// @}
+
  private:
   /** Serialization function */
   friend class boost::serialization::access;
diff --git a/gtsam/hybrid/HybridDiscreteFactor.cpp b/gtsam/hybrid/HybridDiscreteFactor.cpp
index 0455e1e90..605ea5738 100644
--- a/gtsam/hybrid/HybridDiscreteFactor.cpp
+++ b/gtsam/hybrid/HybridDiscreteFactor.cpp
@@ -17,6 +17,7 @@
  */
 
 #include <gtsam/hybrid/HybridDiscreteFactor.h>
+#include <gtsam/hybrid/HybridValues.h>
 
 #include <boost/make_shared.hpp>
 
@@ -50,4 +51,10 @@ void HybridDiscreteFactor::print(const std::string &s,
   inner_->print("\n", formatter);
 };
 
+/* ************************************************************************ */
+double HybridDiscreteFactor::error(const HybridValues &values) const {
+  return -log((*inner_)(values.discrete()));
+}
+/* ************************************************************************ */
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/HybridDiscreteFactor.h b/gtsam/hybrid/HybridDiscreteFactor.h
index 015dc46f8..6e914d38b 100644
--- a/gtsam/hybrid/HybridDiscreteFactor.h
+++ b/gtsam/hybrid/HybridDiscreteFactor.h
@@ -24,10 +24,12 @@
 
 namespace gtsam {
 
+class HybridValues;
+
 /**
- * A HybridDiscreteFactor is a thin container for DiscreteFactor, which allows
- * us to hide the implementation of DiscreteFactor and thus avoid diamond
- * inheritance.
+ * A HybridDiscreteFactor is a thin container for DiscreteFactor, which
+ * allows us to hide the implementation of DiscreteFactor and thus avoid
+ * diamond inheritance.
  *
  * @ingroup hybrid
  */
@@ -59,9 +61,15 @@ class GTSAM_EXPORT HybridDiscreteFactor : public HybridFactor {
       const KeyFormatter &formatter = DefaultKeyFormatter) const override;
 
   /// @}
+  /// @name Standard Interface
+  /// @{
 
   /// Return pointer to the internal discrete factor
   DiscreteFactor::shared_ptr inner() const { return inner_; }
+
+  /// Return the error of the underlying Discrete Factor.
+  double error(const HybridValues &values) const override;
+  /// @}
 };
 
 // traits
diff --git a/gtsam/hybrid/HybridFactor.h b/gtsam/hybrid/HybridFactor.h
index e0cae55c1..a28fee8ed 100644
--- a/gtsam/hybrid/HybridFactor.h
+++ b/gtsam/hybrid/HybridFactor.h
@@ -26,6 +26,8 @@
 #include <string>
 namespace gtsam {
 
+class HybridValues;
+
 KeyVector CollectKeys(const KeyVector &continuousKeys,
                       const DiscreteKeys &discreteKeys);
 KeyVector CollectKeys(const KeyVector &keys1, const KeyVector &keys2);
@@ -110,6 +112,15 @@ class GTSAM_EXPORT HybridFactor : public Factor {
   /// @name Standard Interface
   /// @{
 
+  /**
+   * @brief Compute the error of this Gaussian Mixture given the continuous
+   * values and a discrete assignment.
+   *
+   * @param values Continuous values and discrete assignment.
+   * @return double
+   */
+  virtual double error(const HybridValues &values) const = 0;
+
   /// True if this is a factor of discrete variables only.
   bool isDiscrete() const { return isDiscrete_; }
 
diff --git a/gtsam/hybrid/HybridGaussianFactor.cpp b/gtsam/hybrid/HybridGaussianFactor.cpp
index ba0c0bf1a..5a89a04a8 100644
--- a/gtsam/hybrid/HybridGaussianFactor.cpp
+++ b/gtsam/hybrid/HybridGaussianFactor.cpp
@@ -16,6 +16,7 @@
  */
 
 #include <gtsam/hybrid/HybridGaussianFactor.h>
+#include <gtsam/hybrid/HybridValues.h>
 #include <gtsam/linear/HessianFactor.h>
 #include <gtsam/linear/JacobianFactor.h>
 
@@ -54,4 +55,10 @@ void HybridGaussianFactor::print(const std::string &s,
   inner_->print("\n", formatter);
 };
 
+/* ************************************************************************ */
+double HybridGaussianFactor::error(const HybridValues &values) const {
+  return inner_->error(values.continuous());
+}
+/* ************************************************************************ */
+
 }  // namespace gtsam
diff --git a/gtsam/hybrid/HybridGaussianFactor.h b/gtsam/hybrid/HybridGaussianFactor.h
index 966524b81..897da9caa 100644
--- a/gtsam/hybrid/HybridGaussianFactor.h
+++ b/gtsam/hybrid/HybridGaussianFactor.h
@@ -25,6 +25,7 @@ namespace gtsam {
 // Forward declarations
 class JacobianFactor;
 class HessianFactor;
+class HybridValues;
 
 /**
  * A HybridGaussianFactor is a layer over GaussianFactor so that we do not have
@@ -92,8 +93,15 @@ class GTSAM_EXPORT HybridGaussianFactor : public HybridFactor {
       const KeyFormatter &formatter = DefaultKeyFormatter) const override;
 
   /// @}
+  /// @name Standard Interface
+  /// @{
 
+  /// Return pointer to the internal discrete factor
   GaussianFactor::shared_ptr inner() const { return inner_; }
+
+  /// Return the error of the underlying Discrete Factor.
+  double error(const HybridValues &values) const override;
+  /// @}
 };
 
 // traits
diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.cpp b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
index 5ea677ab5..6af0fb1a9 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.cpp
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
@@ -498,26 +498,8 @@ AlgebraicDecisionTree<Key> HybridGaussianFactorGraph::error(
 /* ************************************************************************ */
 double HybridGaussianFactorGraph::error(const HybridValues &values) const {
   double error = 0.0;
-  for (size_t idx = 0; idx < size(); idx++) {
-    // TODO(dellaert): just use a virtual method defined in HybridFactor.
-    auto factor = factors_.at(idx);
-
-    if (factor->isHybrid()) {
-      if (auto c = boost::dynamic_pointer_cast<HybridConditional>(factor)) {
-        error += c->asMixture()->error(values);
-      }
-      if (auto f = boost::dynamic_pointer_cast<GaussianMixtureFactor>(factor)) {
-        error += f->error(values);
-      }
-
-    } else if (factor->isContinuous()) {
-      if (auto f = boost::dynamic_pointer_cast<HybridGaussianFactor>(factor)) {
-        error += f->inner()->error(values.continuous());
-      }
-      if (auto cg = boost::dynamic_pointer_cast<HybridConditional>(factor)) {
-        error += cg->asGaussian()->error(values.continuous());
-      }
-    }
+  for (auto &factor : factors_) {
+    error += factor->error(values);
   }
   return error;
 }
diff --git a/gtsam/hybrid/HybridNonlinearFactor.h b/gtsam/hybrid/HybridNonlinearFactor.h
index 7776347b3..9b3e780ef 100644
--- a/gtsam/hybrid/HybridNonlinearFactor.h
+++ b/gtsam/hybrid/HybridNonlinearFactor.h
@@ -51,12 +51,22 @@ class HybridNonlinearFactor : public HybridFactor {
       const KeyFormatter &formatter = DefaultKeyFormatter) const override;
 
   /// @}
+  /// @name Standard Interface
+  /// @{
 
   NonlinearFactor::shared_ptr inner() const { return inner_; }
 
+  /// Error for HybridValues is not provided for nonlinear factor.
+  double error(const HybridValues &values) const override {
+    throw std::runtime_error(
+        "HybridNonlinearFactor::error(HybridValues) not implemented.");
+  }
+
   /// Linearize to a HybridGaussianFactor at the linearization point `c`.
   boost::shared_ptr<HybridGaussianFactor> linearize(const Values &c) const {
     return boost::make_shared<HybridGaussianFactor>(inner_->linearize(c));
   }
+
+  /// @}
 };
 }  // namespace gtsam
diff --git a/gtsam/hybrid/MixtureFactor.h b/gtsam/hybrid/MixtureFactor.h
index f29a84022..fc1a9a2b8 100644
--- a/gtsam/hybrid/MixtureFactor.h
+++ b/gtsam/hybrid/MixtureFactor.h
@@ -161,6 +161,12 @@ class MixtureFactor : public HybridFactor {
                              factor, continuousValues);
   }
 
+  /// Error for HybridValues is not provided for nonlinear hybrid factor.
+  double error(const HybridValues &values) const override {
+    throw std::runtime_error(
+        "MixtureFactor::error(HybridValues) not implemented.");
+  }
+
   size_t dim() const {
     // TODO(Varun)
     throw std::runtime_error("MixtureFactor::dim not implemented.");