IH Company of Bitter Iron Mortifactor

[Iron Fella]

Howdy friends. I run a Company of Bitter Iron IH army. This rite of war gives every model with Bitter Duty the Hatred (Traitors) rule and gives you the option to give any independent character in your army the Bitter Duty rule. My question is what happens when you give a Mortifactor in this rite of war Bitter Duty. According to his Keeper of the Dead rule, he is obliged to join the dreadnought, but Bitter Duty seemingly prevents him from joining units that do not also have Bitter Duty. I assume that the solution is that the Mortifactor cannot select to take Bitter Duty as it causes a funny rules error, but wanted to see if a smarter brain than mine can help.

[Pearson73]

My understanding is that characters with Bitter Duty can join units without, but characters without it cannot join those with it.