--- a/src/modbus.c	2022-08-18 09:59:07.516963266 +0800
+++ a/src/modbus.c	2022-08-18 10:02:05.482953900 +0800
@@ -961,7 +961,7 @@
                 nb_write, nb, MODBUS_MAX_WR_WRITE_REGISTERS, MODBUS_MAX_WR_READ_REGISTERS);
         } else if (mapping_address < 0 ||
                    (mapping_address + nb) > mb_mapping->nb_registers ||
-                   mapping_address < 0 ||
+                   mapping_address_write < 0 ||
                    (mapping_address_write + nb_write) > mb_mapping->nb_registers) {
             rsp_length = response_exception(
                 ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,