Why does functional simulation generate Zs?

I have a very simple project, which sends a fixed packet over serial.

I share the files in this github repository

https://github.com/fabiodl/verilogutils/

If I run a functional simulation (Waveform.vwf in the repository) I get some 'Z' in the value of an internal register, see the attached picture.

Nowhere in the code I set anything high impedance. Where is that Z originating from?