diff --git a/report/metrics.go b/report/metrics.go
index 2d0f7e529..44c5a906d 100644
--- a/report/metrics.go
+++ b/report/metrics.go
@@ -24,14 +24,25 @@ func (m Metrics) Merge(other Metrics) Metrics {
 		return m
 	}
 	result := m.Copy()
+	result.UnsafeMerge(other)
+	return result
+}
+
+// UnsafeMerge merges another set of Metrics into m, modifying the original.
+func (m Metrics) UnsafeMerge(other Metrics) {
+	if len(m) == 0 {
+		if len(other) > 0 {
+			m = other.Copy()
+		}
+		return
+	}
 	for k, v := range other {
-		if rv, ok := result[k]; ok {
-			result[k] = rv.Merge(v)
+		if rv, ok := m[k]; ok {
+			m[k] = rv.Merge(v)
 		} else {
-			result[k] = v
+			m[k] = v
 		}
 	}
-	return result
 }
 
 // Copy returns a value copy of the sets map.
diff --git a/report/node.go b/report/node.go
index 9d075217b..045102779 100644
--- a/report/node.go
+++ b/report/node.go
@@ -186,7 +186,7 @@ func (n *Node) UnsafeMerge(other Node) {
 	n.Sets = n.Sets.Merge(other.Sets)
 	n.Adjacency = n.Adjacency.Merge(other.Adjacency)
 	n.Latest = n.Latest.Merge(other.Latest)
-	n.Metrics = n.Metrics.Merge(other.Metrics)
+	n.Metrics.UnsafeMerge(other.Metrics)
 	n.Parents = n.Parents.Merge(other.Parents)
 	n.Children = n.Children.Merge(other.Children)
 }