diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 8fc504c89..f557ceee5 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -522,15 +522,17 @@ hb_buffer_t::merge_clusters_impl (unsigned int start,
     cluster = hb_min (cluster, info[i].cluster);
 
   /* Extend end */
-  while (end < len && info[end - 1].cluster == info[end].cluster)
-    end++;
+  if (cluster != info[end - 1].cluster)
+    while (end < len && info[end - 1].cluster == info[end].cluster)
+      end++;
 
   /* Extend start */
-  while (idx < start && info[start - 1].cluster == info[start].cluster)
-    start--;
+  if (cluster != info[start].cluster)
+    while (idx < start && info[start - 1].cluster == info[start].cluster)
+      start--;
 
   /* If we hit the start of buffer, continue in out-buffer. */
-  if (idx == start)
+  if (idx == start && info[start].cluster != cluster)
     for (unsigned int i = out_len; i && out_info[i - 1].cluster == info[start].cluster; i--)
       set_cluster (out_info[i - 1], cluster);