diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index 8dd4cf4de..4f876d680 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -1235,6 +1235,7 @@ struct PairSet
       buffer->idx = pos;
       return_trace (true);
     }
+    buffer->unsafe_to_concat (buffer->idx, pos + 1);
     return_trace (false);
   }
 
@@ -1640,6 +1641,8 @@ struct PairPosFormat2
     success:
     if (applied_first || applied_second)
       buffer->unsafe_to_break (buffer->idx, skippy_iter.idx + 1);
+    else
+      buffer->unsafe_to_concat (buffer->idx, skippy_iter.idx + 1);
 
     boring: