diff --git a/passes/opt/opt_dff.cc b/passes/opt/opt_dff.cc
index 726516fea..4ed4b0cb6 100644
--- a/passes/opt/opt_dff.cc
+++ b/passes/opt/opt_dff.cc
@@ -737,9 +737,12 @@ struct OptDffWorker
 	bool run_constbits() {
 		ModWalker modwalker(module->design, module);
 		QuickConeSat qcsat(modwalker);
-		std::vector<RTLIL::Cell*> cells_to_remove;
 
-		// Run as a separate sub-pass, so that we don't mutate (non-FF) cells under ModWalker.
+		// Defer mutating cells by removing them/emiting new flip flops so that
+		// cell references in modwalker are not invalidated
+		std::vector<RTLIL::Cell*> cells_to_remove;
+		std::vector<FfData> ffs_to_emit;
+
 		bool did_something = false;
 		for (auto cell : module->selected_cells()) {
 			if (!RTLIL::builtin_ff_cell_types().count(cell->type))
@@ -837,12 +840,14 @@ struct OptDffWorker
 				}
 				ff = ff.slice(keep_bits);
 				ff.cell = cell;
-				ff.emit();
+				ffs_to_emit.emplace_back(ff);
 				did_something = true;
 			}
 		}
 		for (auto* cell : cells_to_remove)
 			module->remove(cell);
+		for (auto& ff : ffs_to_emit)
+			ff.emit();
 		return did_something;
 	}
 };