The automatic missile design sequence for NPR's needs to check it is not building warhead 0 antmissiles as they don't do a lot.
I have reproduced this now. It is a bug in the missile design code but it only affects NPRs with certain levels of warhead tech. The warhead calculation is using 1 / Warhead Strength Tech to determine how much space to devote to the warhead. If the warhead strength tech is 4, then this works out to 0.25 which works fine. For strength 3, it ends up with 0.3333, which means a zero strength warhead. I'll sort it out for the next version
Steve