diff --git a/passes/proc/proc_mux.cc b/passes/proc/proc_mux.cc
index 45fb417c5..aff295f98 100644
--- a/passes/proc/proc_mux.cc
+++ b/passes/proc/proc_mux.cc
@@ -223,6 +223,7 @@ struct MuxGenCtx {
 		sstr << "$procmux$" << (autoidx++);
 
 		RTLIL::Wire *cmp_wire = mod->addWire(sstr.str() + "_CMP", 0);
+		cmp_wire->transfer_src_attribute(sw);
 
 		for (auto comp : *compare)
 		{