diff --git a/passes/opt/opt_merge.cc b/passes/opt/opt_merge.cc
index b46b4bf4b..93d9f012f 100644
--- a/passes/opt/opt_merge.cc
+++ b/passes/opt/opt_merge.cc
@@ -276,6 +276,7 @@ struct OptMergeWorker
 		}
 
 		ct.cell_types.erase("$tribuf");
+		ct.cell_types.erase("$_TBUF_");
 		ct.cell_types.erase("$anyseq");
 		ct.cell_types.erase("$anyconst");