diff --git a/passes/cmds/setundef.cc b/passes/cmds/setundef.cc
index 4ba58036b66..dce33c05bb5 100644
--- a/passes/cmds/setundef.cc
+++ b/passes/cmds/setundef.cc
@@ -243,7 +243,7 @@ struct SetundefPass : public Pass {
 			{
 				for (auto *cell : module->selected_cells()) {
 					for (auto &parameter : cell->parameters) {
-						for (auto bit : parameter.second) {
+						for (auto &bit : parameter.second.bits()) {
 							if (bit > RTLIL::State::S1)
 								bit = worker.next_bit();
 						}