HTTPS support on WIMBOOT

Hi

I could boot into WinPE environment with WIMBOOt on HTTP protocol
But it will display "Could not start download:Operation not supported (http://ipxe.org/3c092083)" when I change to HTTS protocol (change wimboot download URL from http to https)
For example :
Original : kernel http://xxxxxx/wimboot
Current : kernel https://xxxxxx/wimboot

I had add the web certificate during iPXE built.
Might I know if it is caused by WIMBOOT is only support HTTP protocol but not HTTPS ?

Thanks.