diff --git a/passes/opt/opt_expr.cc b/passes/opt/opt_expr.cc
index b26ffc542..07c54c9d0 100644
--- a/passes/opt/opt_expr.cc
+++ b/passes/opt/opt_expr.cc
@@ -409,6 +409,9 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
 		}
 	}
 
+	CellTypes ct_memcells;
+	ct_memcells.setup_stdcells_mem();
+
 	if (!noclkinv)
 	for (auto cell : module->cells())
 	if (design->selected(module, cell)) {
@@ -432,6 +435,9 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
 		if (cell->type.in(ID($dffe), ID($adffe), ID($aldffe), ID($sdffe), ID($sdffce), ID($dffsre), ID($dlatch), ID($adlatch), ID($dlatchsr)))
 			handle_polarity_inv(cell, ID::EN, ID::EN_POLARITY, assign_map, invert_map);
 
+		if (!ct_memcells.cell_known(cell->type))
+			continue;
+
 		handle_clkpol_celltype_swap(cell, "$_SR_N?_", "$_SR_P?_", ID::S, assign_map, invert_map);
 		handle_clkpol_celltype_swap(cell, "$_SR_?N_", "$_SR_?P_", ID::R, assign_map, invert_map);