OTA continuously was Failed

Hi Supporter
I use Ameba26(RTL8720DF), example code use, OTA_Basic, when I OTA(from network port) first time with image1 (address:0x0800B000), output log is success and reboot is OK.But when I OTA(from network port) second time with image2 (address:0x0800B000), output log is success and after reboot the serial log as below

#[MODULE_BOOT-LEVEL_ERROR]:IMG2 Invalid
For this problem, I need to use serial port erase the flash and reupload with serial port.
after erase and upload from serial port and then OTA is success.But OTA continuously with different image is fail.How to prevent this problem?


image