diff --git a/ports/stm32/mboot/dfu.h b/ports/stm32/mboot/dfu.h
index 1f53c1f069..e826e217f0 100644
--- a/ports/stm32/mboot/dfu.h
+++ b/ports/stm32/mboot/dfu.h
@@ -32,6 +32,13 @@
 
 #define DFU_XFER_SIZE (2048)
 
+// The DFU standard allows for error messages to be sent (via str index) in GETSTATUS responses
+#define MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER_IDX 0x10
+#define MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER "Can't overwrite mboot"
+
+#define MBOOT_ERROR_STR_INVALID_ADDRESS_IDX 0x11
+#define MBOOT_ERROR_STR_INVALID_ADDRESS "Address out of range"
+
 // DFU class requests
 enum {
     DFU_DETACH = 0,
diff --git a/ports/stm32/mboot/main.c b/ports/stm32/mboot/main.c
index 134e40a839..b24fa7daa5 100644
--- a/ports/stm32/mboot/main.c
+++ b/ports/stm32/mboot/main.c
@@ -526,6 +526,13 @@ static const flash_layout_t flash_layout[] = {
 
 #endif
 
+static inline bool flash_is_valid_addr(uint32_t addr) {
+    uint8_t last = MP_ARRAY_SIZE(flash_layout) - 1;
+    uint32_t end_of_flash = flash_layout[last].base_address +
+        flash_layout[last].sector_count * flash_layout[last].sector_size;
+    return flash_layout[0].base_address <= addr && addr < end_of_flash;
+}
+
 static uint32_t flash_get_sector_index(uint32_t addr, uint32_t *sector_size) {
     if (addr >= flash_layout[0].base_address) {
         uint32_t sector_index = 0;
@@ -575,6 +582,8 @@ static int flash_page_erase(uint32_t addr, uint32_t *next_addr) {
     uint32_t sector = flash_get_sector_index(addr, &sector_size);
     if (sector == 0) {
         // Don't allow to erase the sector with this bootloader in it
+        dfu_context.status = DFU_STATUS_ERROR_ADDRESS;
+        dfu_context.error = MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER_IDX;
         return -1;
     }
 
@@ -619,6 +628,8 @@ static int flash_page_erase(uint32_t addr, uint32_t *next_addr) {
 static int flash_write(uint32_t addr, const uint8_t *src8, size_t len) {
     if (addr >= flash_layout[0].base_address && addr < flash_layout[0].base_address + flash_layout[0].sector_size) {
         // Don't allow to write the sector with this bootloader in it
+        dfu_context.status = DFU_STATUS_ERROR_ADDRESS;
+        dfu_context.error = MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER_IDX;
         return -1;
     }
 
@@ -732,7 +743,13 @@ int do_write(uint32_t addr, const uint8_t *src8, size_t len) {
     }
     #endif
 
-    return flash_write(addr, src8, len);
+    if (flash_is_valid_addr(addr)) {
+        return flash_write(addr, src8, len);
+    }
+
+    dfu_context.status = DFU_STATUS_ERROR_ADDRESS;
+    dfu_context.error = MBOOT_ERROR_STR_INVALID_ADDRESS_IDX;
+    return -1;
 }
 
 /******************************************************************************/
@@ -909,6 +926,8 @@ uint8_t i2c_slave_process_tx_byte(void) {
 static void dfu_init(void) {
     dfu_context.state = DFU_STATE_IDLE;
     dfu_context.cmd = DFU_CMD_NONE;
+    dfu_context.status = DFU_STATUS_OK;
+    dfu_context.error = 0;
     dfu_context.addr = 0x08000000;
 }
 
@@ -1158,6 +1177,14 @@ static uint8_t *pyb_usbdd_StrDescriptor(USBD_HandleTypeDef *pdev, uint8_t idx, u
             USBD_GetString((uint8_t*)FLASH_LAYOUT_STR, str_desc, length);
             return str_desc;
 
+        case MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER_IDX:
+            USBD_GetString((uint8_t*)MBOOT_ERROR_STR_OVERWRITE_BOOTLOADER, str_desc, length);
+            return str_desc;
+
+        case MBOOT_ERROR_STR_INVALID_ADDRESS_IDX:
+            USBD_GetString((uint8_t*)MBOOT_ERROR_STR_INVALID_ADDRESS, str_desc, length);
+            return str_desc;
+
         default:
             return NULL;
     }