diff --git a/kernel/celltypes.h b/kernel/celltypes.h
index 34a6e56fa..4a600af9d 100644
--- a/kernel/celltypes.h
+++ b/kernel/celltypes.h
@@ -216,7 +216,7 @@ struct CellTypes
 			type = "$shl";
 
 		if (type != "$sshr" && type != "$sshl" && type != "$shr" && type != "$shl" &&
-				type != "$pos" && type != "$neg" && type != "$not") {
+				type != "$pos" && type != "$neg" && type != "$not" && type != "$bu0") {
 			if (!signed1 || !signed2)
 				signed1 = false, signed2 = false;
 		}
diff --git a/passes/opt/opt_const.cc b/passes/opt/opt_const.cc
index ad9a71b15..da71ec30e 100644
--- a/passes/opt/opt_const.cc
+++ b/passes/opt/opt_const.cc
@@ -463,6 +463,7 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
 		FOLD_2ARG_CELL(pow)
 
 		FOLD_1ARG_CELL(pos)
+		FOLD_1ARG_CELL(bu0)
 		FOLD_1ARG_CELL(neg)
 
 		// be very conservative with optimizing $mux cells as we do not want to break mux trees