diff --git a/gtsam/discrete/DiscreteConditional.cpp b/gtsam/discrete/DiscreteConditional.cpp
index 3f0c9f511..5ab0c59ec 100644
--- a/gtsam/discrete/DiscreteConditional.cpp
+++ b/gtsam/discrete/DiscreteConditional.cpp
@@ -259,8 +259,18 @@ size_t DiscreteConditional::argmax(const DiscreteValues& parentsValues) const {
 
 /* ************************************************************************** */
 void DiscreteConditional::sampleInPlace(DiscreteValues* values) const {
-  assert(nrFrontals() == 1);
-  Key j = (firstFrontalKey());
+  // throw if more than one frontal:
+  if (nrFrontals() != 1) {
+    throw std::invalid_argument(
+        "DiscreteConditional::sampleInPlace can only be called on single "
+        "variable conditionals");
+  }
+  Key j = firstFrontalKey();
+  // throw if values already contains j:
+  if (values->count(j) > 0) {
+    throw std::invalid_argument(
+        "DiscreteConditional::sampleInPlace: values already contains j");
+  }
   size_t sampled = sample(*values);  // Sample variable given parents
   (*values)[j] = sampled;            // store result in partial solution
 }
@@ -467,9 +477,7 @@ double DiscreteConditional::evaluate(const HybridValues& x) const {
 }
 
 /* ************************************************************************* */
-double DiscreteConditional::negLogConstant() const {
-  return 0.0;
-}
+double DiscreteConditional::negLogConstant() const { return 0.0; }
 
 /* ************************************************************************* */
 
diff --git a/gtsam/discrete/DiscreteConditional.h b/gtsam/discrete/DiscreteConditional.h
index e16100d0a..f59e29285 100644
--- a/gtsam/discrete/DiscreteConditional.h
+++ b/gtsam/discrete/DiscreteConditional.h
@@ -168,7 +168,7 @@ class GTSAM_EXPORT DiscreteConditional
     static_cast<const BaseConditional*>(this)->print(s, formatter);
   }
 
-  /// Evaluate, just look up in AlgebraicDecisonTree
+  /// Evaluate, just look up in AlgebraicDecisionTree
   double evaluate(const DiscreteValues& values) const {
     return ADT::operator()(values);
   }