diff --git a/cpp/src/hdbscan/runner.h b/cpp/src/hdbscan/runner.h
index ff7ec71f66..bffa0777af 100644
--- a/cpp/src/hdbscan/runner.h
+++ b/cpp/src/hdbscan/runner.h
@@ -71,7 +71,13 @@ struct FixConnectivitiesRedOp {
     if (rit < m && a.key > -1 && colors[rit] != colors[a.key]) {
       value_t core_dist_rit = core_dists[rit];
       value_t core_dist_a = max(core_dist_rit, max(core_dists[a.key], a.value));
-      value_t core_dist_b = max(core_dist_rit, max(core_dists[b.key], b.value));
+
+      value_t core_dist_b;
+      if (b.key > -1) {
+        core_dist_b = max(core_dist_rit, max(core_dists[b.key], b.value));
+      } else {
+        core_dist_b = b.value;
+      }
 
       return core_dist_a < core_dist_b ? KVP(a.key, core_dist_a)
                                        : KVP(b.key, core_dist_b);