diff --git a/passes/cmds/dft_tag.cc b/passes/cmds/dft_tag.cc
index ba7d14323..b6afe6f89 100644
--- a/passes/cmds/dft_tag.cc
+++ b/passes/cmds/dft_tag.cc
@@ -883,7 +883,7 @@ struct DftTagWorker {
 	{
 		if (sig_a.is_fully_const()) {
 			auto const_val = sig_a.as_const();
-			for (auto bit : const_val)
+			for (State& bit : const_val.bits())
 				bit = bit == State::S0 ? State::S1 : bit == State::S1 ? State::S0 : bit;
 			return const_val;
 		}