diff --git a/render/render.go b/render/render.go
index 1ba43f48c..eb3b6c8a2 100644
--- a/render/render.go
+++ b/render/render.go
@@ -169,6 +169,7 @@ func (ret *joinResults) mapChild(from, to string) {
 }
 
 // Add m into the results as a top-level node, mapped from original ID
+// Note it is not safe to mix calls to add() with addChild(), addChildAndChildren() or addUnmappedChild()
 func (ret *joinResults) add(from string, m report.Node) {
 	if existing, ok := ret.nodes[m.ID]; ok {
 		m = m.Merge(existing)
@@ -203,7 +204,7 @@ func (ret *joinResults) addChild(m report.Node, id string, topology string) {
 func (ret *joinResults) addChildAndChildren(m report.Node, id string, topology string) {
 	ret.addUnmappedChild(m, id, topology)
 	result := ret.nodes[id]
-	result.Children = result.Children.Merge(m.Children)
+	result.Children.UnsafeMerge(m.Children)
 	ret.nodes[id] = result
 	ret.mapChild(m.ID, id)
 }
diff --git a/report/node_set.go b/report/node_set.go
index eef713748..e236bd3db 100644
--- a/report/node_set.go
+++ b/report/node_set.go
@@ -72,6 +72,13 @@ func (n NodeSet) Delete(ids ...string) NodeSet {
 	return NodeSet{result}
 }
 
+// UnsafeMerge combines the two NodeSets, altering n
+func (n *NodeSet) UnsafeMerge(other NodeSet) {
+	other.psMap.ForEach(func(key string, otherVal interface{}) {
+		n.psMap = n.psMap.UnsafeMutableSet(key, otherVal)
+	})
+}
+
 // Merge combines the two NodeSets and returns a new result.
 func (n NodeSet) Merge(other NodeSet) NodeSet {
 	nSize, otherSize := n.Size(), other.Size()