[Q] CM12 fails with message : failed to verify whole file signature topic

Can some help me to install CM12 on Samsung S4 At&t 1337.

I tried both using TWRP and ROM manger, at the end it gives a message saying : failed to verify whole file signature and aborts and restart the phone back.

Can someone give some pointers on how to by pass the signature validation during installing CM12 on S4 At&t.

thanks,
Neon