diff --git a/passes/cmds/setundef.cc b/passes/cmds/setundef.cc
index f6949c820..5b43d528c 100644
--- a/passes/cmds/setundef.cc
+++ b/passes/cmds/setundef.cc
@@ -404,22 +404,29 @@ struct SetundefPass : public Pass {
 					initwires.insert(wire);
 				}
 
-				for (int wire_types = 0; wire_types < 2; wire_types++)
-					for (auto wire : module->wires())
-					{
-						if (wire->name[0] == (wire_types ? '\\' : '$'))
-					next_wire:
-							continue;
+				for (int wire_types = 0; wire_types < 2; wire_types++) {
+                                        pool<SigBit> ffbitsToErase;
+                                        for (auto wire : module->wires()) {
+                                                if (wire->name[0] == (wire_types ? '\\' : '$')) {
+                                                        next_wire:
+                                                        continue;
+                                                }
 
-						for (auto bit : sigmap(wire))
-							if (!ffbits.count(bit))
-								goto next_wire;
+                                                for (auto bit : sigmap(wire))
+                                                        if (!ffbits.count(bit)) {
+                                                                goto next_wire;
+                                                        }
 
-						for (auto bit : sigmap(wire))
-							ffbits.erase(bit);
+                                                for (auto bit : sigmap(wire)) {
+                                                        ffbitsToErase.insert(bit);
+                                                }
 
-						initwires.insert(wire);
-					}
+                                                initwires.insert(wire);
+                                        }
+                                        for (const auto &bit : ffbitsToErase) {
+                                                ffbits.erase(bit);
+                                        }
+                                }
 
 				for (auto wire : initwires)
 				{