I wanted to build heads just to see how this works. No extra configurations.
So I've been following the documentation and the build system works very well.
I got to the point of running build_iso_dist and it went on for a while without errors until building the kernel.

[D] build_kernel_amd64
 [W] build_kernel_amd64(): required setting is blank: kernel_heads_git
 [W] build_kernel_amd64(): required setting is blank: kernel_heads_flavor
 [E] error in: build_kernel_amd64
 [W] called in: build_kernel_amd64
 [W] called in: umounted sys
 [W] called in: chrooting to execute dpkgdivert...

I looked through all the config files but can't figure out how to set these parameters.

Does anyone know how to fix this?