diff --git a/gtsam/inference/JunctionTree-inst.h b/gtsam/inference/JunctionTree-inst.h
index 5735a3bd2..232246d5e 100644
--- a/gtsam/inference/JunctionTree-inst.h
+++ b/gtsam/inference/JunctionTree-inst.h
@@ -73,7 +73,8 @@ struct ConstructorTraversalData {
 
     // Do symbolic elimination for this node
     SymbolicFactors symbolicFactors;
-    symbolicFactors.reserve(ETreeNode->factors.size() + myData.childSymbolicFactors.size());
+    symbolicFactors.reserve(
+        ETreeNode->factors.size() + myData.childSymbolicFactors.size());
     // Add ETree node factors
     symbolicFactors += ETreeNode->factors;
     // Add symbolic factors passed up from children
@@ -96,29 +97,42 @@ struct ConstructorTraversalData {
 
     // Merge our children if they are in our clique - if our conditional has
     // exactly one fewer parent than our child's conditional.
-    size_t myNrFrontals = 1;
     const size_t myNrParents = myConditional->nrParents();
-    assert(node->newChildren.size() == childConditionals.size());
+    const size_t nrChildren = node->children.size();
+    assert(childConditionals.size() == nrChildren);
 
     gttic(merge_children);
-    // First count how many keys, factors and children we'll end up with
+
+    // decide which children to merge, as index into children
+    std::vector<bool> merge(nrChildren, false);
+    {
+      size_t myNrFrontals = 1;
+      for (size_t i = 0; i < nrChildren; ++i) {
+        // Check if we should merge the i^th child
+        if (myNrParents + myNrFrontals == childConditionals[i]->nrParents()) {
+          sharedNode child = node->children[i];
+          // Increment number of frontal variables
+          myNrFrontals += child->orderedFrontalKeys.size();
+          merge[i] = true;
+        }
+      }
+    }
+
+    // Count how many keys, factors and children we'll end up with
     size_t nrKeys = node->orderedFrontalKeys.size();
     size_t nrFactors = node->factors.size();
-    size_t nrChildren = 0;
+    size_t nrNewChildren = 0;
     // Loop over children
-    for (size_t i = 0; i < childConditionals.size(); ++i) {
-      // Check if we should merge the i^th child
-      if (myNrParents + myNrFrontals == childConditionals[i]->nrParents()) {
+    for (size_t i = 0; i < nrChildren; ++i) {
+      if (merge[i]) {
         // Get a reference to the i, adjusting the index to account for children
         // previously merged and removed from the i list.
         sharedNode child = node->children[i];
         nrKeys += child->orderedFrontalKeys.size();
         nrFactors += child->factors.size();
-        nrChildren += child->children.size();
-        // Increment number of frontal variables
-        myNrFrontals += child->orderedFrontalKeys.size();
+        nrNewChildren += child->children.size();
       } else {
-        nrChildren += 1; // we keep the child
+        nrNewChildren += 1; // we keep the child
       }
     }
 
@@ -126,14 +140,14 @@ struct ConstructorTraversalData {
     node->orderedFrontalKeys.reserve(nrKeys);
     node->factors.reserve(nrFactors);
     typename Node::Children newChildren;
-    newChildren.reserve(nrChildren);
-    myNrFrontals = 1;
-    int combinedProblemSize = (int) (myConditional->size() * symbolicFactors.size());
+    newChildren.reserve(nrNewChildren);
+    int combinedProblemSize = (int) (myConditional->size()
+        * symbolicFactors.size());
     // Loop over newChildren
-    for (size_t i = 0; i < childConditionals.size(); ++i) {
+    for (size_t i = 0; i < nrChildren; ++i) {
       // Check if we should merge the i^th child
       sharedNode child = node->children[i];
-      if (myNrParents + myNrFrontals == childConditionals[i]->nrParents()) {
+      if (merge[i]) {
         // Get a reference to the i, adjusting the index to account for newChildren
         // previously merged and removed from the i list.
         // Merge keys. For efficiency, we add keys in reverse order at end, calling reverse after..
@@ -141,18 +155,21 @@ struct ConstructorTraversalData {
             child->orderedFrontalKeys.rbegin(),
             child->orderedFrontalKeys.rend());
         // Merge keys, factors, and children.
-        node->factors.insert(node->factors.end(), child->factors.begin(), child->factors.end());
-        newChildren.insert(newChildren.end(), child->children.begin(), child->children.end());
+        node->factors.insert(node->factors.end(), child->factors.begin(),
+            child->factors.end());
+        newChildren.insert(newChildren.end(), child->children.begin(),
+            child->children.end());
         // Increment problem size
-        combinedProblemSize = std::max(combinedProblemSize, child->problemSize_);
+        combinedProblemSize = std::max(combinedProblemSize,
+            child->problemSize_);
         // Increment number of frontal variables
-        myNrFrontals += child->orderedFrontalKeys.size();
       } else {
         newChildren.push_back(child); // we keep the child
       }
     }
     node->children = newChildren;
-    std::reverse(node->orderedFrontalKeys.begin(), node->orderedFrontalKeys.end());
+    std::reverse(node->orderedFrontalKeys.begin(),
+        node->orderedFrontalKeys.end());
     gttoc(merge_children);
     node->problemSize_ = combinedProblemSize;
   }